Ideal rifle for ww2?

[Airframes]

Just to clarify for Michael (Parsifal) - the Lee-Enfield No.4, in its various versions, started to replace the WW1 Short, Magazine, Lee- Enfield (SMLE) rifle in 1941 in British and Commonwealth military service, although the SMLE still saw service in second-line units. The SMLE was virtually hand made, and tailored to the firere, with three different butt lengths available.
The No4 theoretically was available in 'short' and long' but length, but this seems rare!
In 1956, the British army finally took the decision to go 'semi-auto' and, after very lengthy trials of various weapons, including one, which, in much developed form, eventually became the current UK rifle, opted for the FAL design which, built under licence to British requirements, became the L1A1 'SLR' (Self Loading Rifle), and entered general service in 1958, remaining the 'front line' weapon until the mid 1980s, when the current 'SA80' L-85 'bullpup' rifle started to enter service, although the SLR remained for some time with second-line units.
I would have thought that NZ and Australian Forces would have had the No4 rifle (and/or the No.5 'Jungle Carbine') up to around 1958, when the SLR came into service, as most of the SMLEs were given, or sold, to former 'Empire' nations, in the Asian and African continents, or sold on the private market.

 

[fastmongrel]

Just to clarify for Michael (Parsifal) - the Lee-Enfield No.4, in its various versions, started to replace the WW1 Short, Magazine, Lee- Enfield (SMLE) rifle in 1941 in British and Commonwealth military service, although the SMLE still saw service in second-line units. The SMLE was virtually hand made, and tailored to the firere, with three different butt lengths available.
The No4 theoretically was available in 'short' and long' but length, but this seems rare!
In 1956, the British army finally took the decision to go 'semi-auto' and, after very lengthy trials of various weapons, including one, which, in much developed form, eventually became the current UK rifle, opted for the FAL design which, built under licence to British requirements, became the L1A1 'SLR' (Self Loading Rifle), and entered general service in 1958, remaining the 'front line' weapon until the mid 1980s, when the current 'SA80' L-85 'bullpup' rifle started to enter service, although the SLR remained for some time with second-line units.
I would have thought that NZ and Australian Forces would have had the No4 rifle (and/or the No.5 'Jungle Carbine') up to around 1958, when the SLR came into service, as most of the SMLEs were given, or sold, to former 'Empire' nations, in the Asian and African continents, or sold on the private market.

The Aussies never got the No4 as far as I know they had the smelly MkIII made at the Australian Lithgow Small Arms factory till they went to the SLR at the same time as the British Army.

 

[stona]

as most of the SMLEs were given, or sold, to former 'Empire' nations, in the Asian and African continents, or sold on the private market.

I saw plenty of them in Nigeria in the 1970s,post Biafra.
I even saw a few toted by what I assume were either soldiers or some kind of policemen,guarding tourist sites a few years ago in India.
I suppose millions of them were manufactured throughout the Commonwelth/Empire.
Cheers
Steve

 

[parsifal]

The Aussies never got the No4 as far as I know they had the smelly MkIII made at the Australian Lithgow Small Arms factory till they went to the SLR at the same time as the British Army.

yes, correct, and we never used the cut down carbine version either, which was a bit of a disaster I have read.

Our Lee Enfields even by my generation, who are SLP men through and through, basically worshipped the SMLE. And they were a joy to learn to shoot with. IIRC we even had a ).22 barrelled version, which actually fired really well....no kick on a heavy gun....perfect.

 

[yulzari]

Not a major point but the design of the Lee-Enfield stock grip was designed to give a straight line grip as a half pike for using the bayonet. Proper bayonet that is. Not the pointy nail pig sticker thing. In a (very) mild defence of which, it was intended to allow the swift withdrawl of the bayonet from the victim. Thus avoiding having to wave your half pike about with a very cross human stuck on the end.

All very Edwardian like the volley sight so that your company could create a long distance beaten zone in lieu of an HMG or the cut off whereby you loaded rounds individually until you had then order 'rapid fire' at which you could open the cut off and use the full magazine.

 

[fastmongrel]

The No4 pig sticker bayonet was very carefully designed and as a bayonet was excellent. It didnt stick in bone, did a lot of damage because a round hole that stays open is worse than a slit that closes up and because it was shorter and lighter than a knife bayonet it didnt ruin the aim when fitted. Unfortunately the designers forgot that a soldier doesnt use his bayonet to stab people very often and a pointy stick is useless for 99% of the tasks a soldier uses a bayonet for, like opening tincans, cutting firewood and most importantly digging small holes in the ground behind bushes.

 

[Bernhart]

heard the pig sticker was good for holding candles too

 

[stug3]

I agree with Matt308 about the SKS, the Russians just missed the boat by a few years (also the AK-47 sort of, but not as close). It would be better with peep sights, but what the hell, it works.

 

[Hansie Bloeckmann]

If, as Garand had wanted, the M-1 had been produced with a lighter cartridge then the 30.06 and with a detachable Mag (10-20 rds?) instead of the clip it would have been more effective.
What if the US as adopted to Brit. .303? Would have made things a lot easier.

Correct: The initial caliber was .276--but as the BAR, Springfield 1903, some M1917 Enfields and the 1919A-1 Browning machine guns were already in std. 30-06 caliber . John Garand was "persuaded" by the Army Ordnance Dept. to build his prototype in .30-Gov't-06 Caliber. The Garand is heavy, kicks like a constipated mule, and the 8 rd. enbloc clip and coupled with the reloading with the "M-1 thumb" hazard, not too hard to overcome on the firing range, but a possible problem in a combat situation (mud, sand, snow, rain)--

Field & Stream rifle guru David Petzal once wrote about a neighbor, where he grew up in NJ - a surviving WW2 Veteran, "Big Red One"--and somewhere in the French bocage area in 1944- was charged by a huge German Feldwebel (Sgt.) carrying the MP-40, which was apparently empty, as he heard the click of the pin falling on an empty chamber- at close range he emptied 15 rounds into the German, none seemed to have any effect , a buddy pulled his 1911-A-1 .45 issue pistol, shot the German right between the eyes at about arm's length-- end of story. Our Army Ordnance thought the M-1 carbine with its light load would be the right weapon to augment the heavier Garand in combat scenarios.

I have no Army nor ordnance background, like others who post on this website-but I wondered why the Ranger team in the movie "Saving Pvt. Ryan" didn't all carry Garands, instead of the Sgt. (Tom Sizemore) with a M-1 carbine, the Brooklyn kid , with the BAR-- they would all have commonality of issue ammo, except for Tom Hank's character, with the Thompson SMG in 45 ACP, I also wondered why, when the lost the big Pfc. in the rain storm scene with the French civilians, as they were not set for air-drop re-supply on ammo, etc- they left the dead Ranger with his Garand and ammo belt, albiet they covered his body with a blanket. Tom Hanks did remove the dog tags, but the ammo could be used by the living to stay alive, I might think.

 

[The Basket]

Ideal? Wow. That's a head scratcher.
Obviously a semi but they are expensive to build and still doubts over long term reliability especially in gunk.
But if you look at the industrial power of say Japan or Italy and could they have built any garand type rifle in huge numbers quickly? No?
One thing is that semi autos were about for many years before ww2 and this idea that the Garand was revolutionary is not true. The Mexicans and the French has service semi autos which served in ww1.

Individual rifles are of no concern so what you need is a rifle which can be built cheaply enough and quickly enough so you can equip your army in time of war based on what is possible. So will differ from country to country.

 

[fastmongrel]

Individual rifles are of no concern so what you need is a rifle which can be built cheaply enough and quickly enough so you can equip your army in time of war based on what is possible. So will differ from country to country.

The MAS40 French self loader has been described as the best rifle of WW2 even though only a handful saw service right at the end of the war. If Germany hadnt invaded in May 1940 it would have gone into full production in June 1940. I have read that compared to a Garand it cost half as much, had half the parts and needed a third of the machining time, it was only about 15% more expensive than the MAS36 bolt action with which it shared many parts like barrel and stock.

 

[Ivan1GFP]

Getting back to the original question "IDEAL rifle" and not best substitute or best available.

6.5mm cartridge with a 120-125 grain bullet (8 grams) spitzer boat tail at 2600fps (788ms).

Weight about 4-4.5 kg

Pretty much a FN FAL

20 round magazines although a 30 might be able to fit (or be interchangeable with the squad LMG?)

Full auto depends on how controllable it is. With about 70% of the recoil of a 7.62 nato round it has a chance even if heavier recoil than the true intermediates.

There is nothing in the FN FAL that didn't exist in WW II except perhaps for the exact metallurgy which could be solved by making it just a bit heavier ( although with a bit small receiver it may be a wash?)

Front sight an interchangeable blade to allow for zeroing ( various heights and the ability to be moved side ways) protected by wings.

Rear sight mounted on the back end of the receiver with elevation steps of 200, 300, 400 and 500 yds or meters depending on country. One or two steps each side for wind correction nice but not essential.

Hello Shortfound6, et al.

It really helps to read what has already been posted in a forum before posting something.
Why am I not surprised that most of what I was about to write was already covered by you several; years ago?


When thinking of a new rifle design, the question is how much of current knowledge and technology can be assumed to be available before or during WW2. Many materials that we use today were obviously not developed yet, but can we use the knowledge we have now that designers back then did not have? I think I will play a little loose with the rules here because no one has actually stated any yet.

We are in almost complete agreement about caliber:
I would choose a 6.5 mm with about a 140-150 grain bullet (A little heavier than your choice for better long range ballistics).
Ideally the cartridge case would be fairly short probably in the 45 mm to 50 mm range depending on what powder capacity needed to be to push a bullet to around 2600-2650 fps. Double base powders would offer more power in a smaller cartridge case as compared to the typical single base powders used in the military .30-06 of the time.
Basically this would be a ballistic twin of the 6.5 mm x 55 Swedish Mauser but in a more compact package and with a sharper case shoulder for good case volume.
There would be a second "standard" loading with a heavier bullet perhaps at a slightly lower velocity for longer ranged weapons such as a LMG but there would be no significant power level differences so that either cartridge could be used interchangeably in any of the weapons.
This is the same idea as the .30 M1 with a 170-something grain bullet, the M72 Match round with a 173 grain bullet and the .30 M2 round with a 152 grain bullet.
All would be boat tail spritzers.

The FAL is a cool design, but my preference would be something with a gas system closer to that of the SVD Dragunov and with a multiple lug rotating bolt. Ideally at least three locking lugs as on the Dragunov.
Front sight would be similar to the M1 Garand or M14 with adjustability for Windage.
Rear sight would probably be a clone of a late model M1 Garand which is pretty hard to beat or perhaps the micrometer style sight that is found on some No.4 Lee Enfield's but with provision for Windage adjustment.

More details on the gas system:
The White gas cutoff system as implemented on the M14 is an interesting idea but did not seem to work quite that well in practice.
It may actually self regulate, but seems to have a very limited range.
The manually regulated system of the FN FAL / SAFN or FN-49 is not bad but would require more work to set. I have personally run into more functioning issues with this system than with the M14 types. This feature was also partly responsible tor the failure of the FAL / T48 in US arctic tests.

Trigger would be similar to the FN-49 / M1 / M14. The FAL generally has a pretty horrible trigger. My own preference is also for a two stage trigger instead of single stage.

The Flash Suppressor / Muzzle Brake could be some kind of design that would reduce recoil and compensate for muzzle climb under full automatic fire.

Thoughts?

- Ivan.

 

[Hansie Bloeckmann]

Oops, your right. Did not look closely enough. Have not had much to do with the US 45, automatically assumed it to be the Browning.

Browning designed the 1911 in 1908- in 1911 the US Army Ordnance Board approved it, and Colt was the first manufacturer of this pistol, in .45ACP Automatic Colt Pistol-great weapon yet today.

 

[Hansie Bloeckmann]

Hello Shortfound6, et al.

It really helps to read what has already been posted in a forum before posting something.
Why am I not surprised that most of what I was about to write was already covered by you several; years ago?


When thinking of a new rifle design, the question is how much of current knowledge and technology can be assumed to be available before or during WW2. Many materials that we use today were obviously not developed yet, but can we use the knowledge we have now that designers back then did not have? I think I will play a little loose with the rules here because no one has actually stated any yet.

We are in almost complete agreement about caliber:
I would choose a 6.5 mm with about a 140-150 grain bullet (A little heavier than your choice for better long range ballistics).
Ideally the cartridge case would be fairly short probably in the 45 mm to 50 mm range depending on what powder capacity needed to be to push a bullet to around 2600-2650 fps. Double base powders would offer more power in a smaller cartridge case as compared to the typical single base powders used in the military .30-06 of the time.
Basically this would be a ballistic twin of the 6.5 mm x 55 Swedish Mauser but in a more compact package and with a sharper case shoulder for good case volume.
There would be a second "standard" loading with a heavier bullet perhaps at a slightly lower velocity for longer ranged weapons such as a LMG but there would be no significant power level differences so that either cartridge could be used interchangeably in any of the weapons.
This is the same idea as the .30 M1 with a 170-something grain bullet, the M72 Match round with a 173 grain bullet and the .30 M2 round with a 152 grain bullet.
All would be boat tail spritzers.

The FAL is a cool design, but my preference would be something with a gas system closer to that of the SVD Dragunov and with a multiple lug rotating bolt. Ideally at least three locking lugs as on the Dragunov.
Front sight would be similar to the M1 Garand or M14 with adjustability for Windage.
Rear sight would probably be a clone of a late model M1 Garand which is pretty hard to beat or perhaps the micrometer style sight that is found on some No.4 Lee Enfield's but with provision for Windage adjustment.

More details on the gas system:
The White gas cutoff system as implemented on the M14 is an interesting idea but did not seem to work quite that well in practice.
It may actually self regulate, but seems to have a very limited range.
The manually regulated system of the FN FAL / SAFN or FN-49 is not bad but would require more work to set. I have personally run into more functioning issues with this system than with the M14 types. This feature was also partly responsible tor the failure of the FAL / T48 in US arctic tests.

Trigger would be similar to the FN-49 / M1 / M14. The FAL generally has a pretty horrible trigger. My own preference is also for a two stage trigger instead of single stage.

The Flash Suppressor / Muzzle Brake could be some kind of design that would reduce recoil and compensate for muzzle climb under full automatic fire.

Thoughts?

- Ivan.

I think you mean: Boat tail spitzers- spritzers add "kick" to alcoholic drinks- or something like that. I like the Speer 168 grain Boat-Tail design bullets- spitzers to the tenth power--Hansie

 

[Ivan1GFP]

Browning designed the 1911 in 1908- in 1911 the US Army Ordnance Board approved it, and Colt was the first manufacturer of this pistol, in .45ACP Automatic Colt Pistol-great weapon yet today.

Hello Hansie Bloeckmann,

M1911A1 is definitely one of my favourites. If you have worked on them you probably have found some of the really goofy issues with these guns. There are about two or three areas of these guns I believe are really poorly designed:
1. The two piece Feed Ramp. There needs to be an overlap between frame ramp and barrel ramp which means that the ramp angles need to be very steep or the case head would be unsupported.
2. The Extractor is located in such a manner that without modification, it pulls the spent case directly against the right side wall of the Ejection Port.
Also, the Ejection Port is a little on the small side for clearing a loaded round with some configurations of bullets.
The lowering and extending of the ejection port is one of the more common modifications.
The Ejector is also often replaced and does help with more reliable ejection.
3. The Slide Stop has very little overlap with the typical Magazine Follower but has no room to extend further because a Round Nose bullet may contact it. Sometimes with not so tightly spec'ed guns, the follower rides past the slide stop.

There are a few other little things such as issues replacing the front sight but those are not really inherent problems in the design.

....Spritzer bullets! I can't stop laughing.
Thanks!

- Ivan.

 

[The Basket]

You would have to do this country to country. Italy was certainly not in a position to make whizz bang new rifles.

 

[Hansie Bloeckmann]

Hello Hansie Bloeckmann,

M1911A1 is definitely one of my favourites. If you have worked on them you probably have found some of the really goofy issues with these guns. There are about two or three areas of these guns I believe are really poorly designed:
1. The two piece Feed Ramp. There needs to be an overlap between frame ramp and barrel ramp which means that the ramp angles need to be very steep or the case head would be unsupported.
2. The Extractor is located in such a manner that without modification, it pulls the spent case directly against the right side wall of the Ejection Port.
Also, the Ejection Port is a little on the small side for clearing a loaded round with some configurations of bullets.
The lowering and extending of the ejection port is one of the more common modifications.
The Ejector is also often replaced and does help with more reliable ejection.
3. The Slide Stop has very little overlap with the typical Magazine Follower but has no room to extend further because a Round Nose bullet may contact it. Sometimes with not so tightly spec'ed guns, the follower rides past the slide stop.

There are a few other little things such as issues replacing the front sight but those are not really inherent problems in the design.

....Spritzer bullets! I can't stop laughing.
Thanks!

- Ivan.

Just wondering- is the 2. listing- Extractor location sometimes the cause of "stove-piping"?? I do not own a 1911-A-1 or any of its many "clones" available in today's market. I have a S&W M39 in 9mm-lightweight- and if you release the magazine, with a round in the chamber, there is an "interceptor" than dis-connects the firing linking- the magazine, whether empty of full, has to be firmly seated before you can fire the weapon. Why S&W designed this, I do not know. A hunting pal in CO. once told me the the 1911-A-1 has 4 distinct safety modes, and his theory was, as the US Army was still a cavalry based force in 1911- there was concern by the Army that a trooper could fire the weapon as he unholstered it, putting a bullet into his horse- I shudder to think of that scenario..

 

[Ivan1GFP]

Just wondering- is the 2. listing- Extractor location sometimes the cause of "stove-piping"?? I do not own a 1911-A-1 or any of its many "clones" available in today's market. I have a S&W M39 in 9mm-lightweight- and if you release the magazine, with a round in the chamber, there is an "interceptor" than dis-connects the firing linking- the magazine, whether empty of full, has to be firmly seated before you can fire the weapon. Why S&W designed this, I do not know. A hunting pal in CO. once told me the the 1911-A-1 has 4 distinct safety modes, and his theory was, as the US Army was still a cavalry based force in 1911- there was concern by the Army that a trooper could fire the weapon as he unholstered it, putting a bullet into his horse- I shudder to think of that scenario..

Hello Hansie Bloeckmann,
I believe that the extractor location along with the right wall of the ejection port ALONG WITH the short ejector is a cause of stove piping.
There needs to be a fair amount of extractor tension against the loaded cartridge / case rim and if there is not, it can release prematurely before it is clear o the gun.
Unmodified, the bottom edge of the extractor is pretty sharp and may hang (in theory) on the case rim.
It also pulls the case to the right directly against the right wall of the ejection port where it deflects up and out.
I have to look to see if I have an unmodified military style ejection port on one of these guns, but I don't believe I do.
The Ejector in my opinion should be longer than stock but not so long as the replacement parts come from the manufacturer.
There is a good balance between longer with enough energy to kick the spent case out and too long as to hang up on trying to eject a live round. There is a lot of manufacturing variation, so this is a part that needs to be cut to length and for reliability a touch shorter is better than a touch too long.

I am actually pretty familiar with the S&W Model 39 as well. I really like the balance and pointability of this gun.
Many years ago, a friend of mine told me that he choice of a defence gun would be the M39 which I found to be an odd choice because I knew he had a lot to choose from. When I asked him why, he told me about the handling and let me try it out. I was hooked.
It is a pity but the later single stack S&W 9 mm don't seem to point quite as well though the triggers are better.

Regarding other issues with the M1911:
My own preference is to have a full length guide rod for the recoil spring.
I also like to have a "shock buffer" to take some of the impact from the slide against the spring guide.
I also like to have the ability to pull the slide back to release the slide stop after reloading when the slide is locked back.
The problem is that there is quite a lot of variation in the manufacturing of the slide stops and slide stop notches so that what works in one gun will not work in another when a shock buffer is in place.
A friend of mine had a gun (I think it was a AMT) that had fit problems with the grip safety because his frame was dimensioned incorrectly. Under my direction, he had to do a lot of adjustments to some parts, especially the 3 leaf spring behind the mainspring housing to get it to function. I still didn't like the result, but it worked.
I have also seen a description of another manufacturer that puts the disconnect notch in the wrong place so that the gun can fire even when not fully in battery.
I have seen a couple Charles Daly guns that had the feed ramps fit wrong. These were new guns and of the ones on the table at a Gun Show, the assembly was different between them in critical areas.... like the feed ramp.

You would have to do this country to country. Italy was certainly not in a position to make whizz bang new rifles.

Hello The Basket,
There are plenty of countries that really had no hope of building anything like what I was describing.
Finland and China come to mind.

- Ivan.

 

[Shortround6]

Hello Shortfound6, et al.

It really helps to read what has already been posted in a forum before posting something.
Why am I not surprised that most of what I was about to write was already covered by you several; years ago?


When thinking of a new rifle design, the question is how much of current knowledge and technology can be assumed to be available before or during WW2. Many materials that we use today were obviously not developed yet, but can we use the knowledge we have now that designers back then did not have? I think I will play a little loose with the rules here because no one has actually stated any yet.

We are in almost complete agreement about caliber:
I would choose a 6.5 mm with about a 140-150 grain bullet (A little heavier than your choice for better long range ballistics).
Ideally the cartridge case would be fairly short probably in the 45 mm to 50 mm range depending on what powder capacity needed to be to push a bullet to around 2600-2650 fps. Double base powders would offer more power in a smaller cartridge case as compared to the typical single base powders used in the military .30-06 of the time.
Basically this would be a ballistic twin of the 6.5 mm x 55 Swedish Mauser but in a more compact package and with a sharper case shoulder for good case volume.
There would be a second "standard" loading with a heavier bullet perhaps at a slightly lower velocity for longer ranged weapons such as a LMG but there would be no significant power level differences so that either cartridge could be used interchangeably in any of the weapons.
This is the same idea as the .30 M1 with a 170-something grain bullet, the M72 Match round with a 173 grain bullet and the .30 M2 round with a 152 grain bullet.
All would be boat tail spritzers.

For use in full auto you need to reduce recoil somewhat, how much is subject to argument but since recoil is proportional to momentum (weight/mass X velocity) and not energy you have two choices, cut bullet weight or cut velocity (or both). Keeping 140-150 grain Spitzers (with or without boat tails) doesn't cut the recoil enough to give you a controllable weapon in full auto. Please remember that the US M2 Ball .30-06 used a 150 grain bullet and the Nato 7.62 used a 147-150 grain bullet. Nobody made a controllable full auto rifle in either cartridge. I don't know if cutting recoil by 20-25% is enough but it is a much better start than cutting velocity by only 10% or so.

In 6.5mm a 120 grain bullet will give (given the same shape) as good or better ballistics than a 150-168 grain .30 cal bullet. A 140 grain 6.5mm bullet acts like a 190-200 grain .30 cal.
You don't need the heavy 6.5mm bullets (which also require quicker twist rifling) unless you are shooting to distances beyond the normal employment ranges of a bipod mounted machine gun.

The US adopted the .30 M1 with a 170-something grain bullet for use in water cooled 1917 Brownings where it increased the max range by something like 1500-2000 yds. Effective range was increased but not by quite as much. Please note this was for indirect fire before 81mm mortars became standard issue.

Our Ideal "rifle" for squad use has no such need for that kind of capability.

I am not too sure how well sharp shoulders really feed in automatic weapons. In any case any advantages they have are mostly theoretical and usually will be unnoticed in a military grade rifle. After I had my 6.5 X .308 for a few years (bolt action) a fellow who shot on some of the teams I did had a rifle built in 6.5mm X .308 Ackley Improved (blown out, sharp shoulder) and he gave me an empty case. Weighing his case and mine empty and full of water to the top of the case mouth his case held slightly more than 1 grain of water more, out of just over 60 grains. Seating a bullet cuts in the capacity quite a bit, but a 2-3% increase in powder capacity doesn't really mean much and a military weapon is much better served with a slightly lower pressure level than going for that last 2% in performance. Short and fat has gotten some good results in bench rest shooting but please remember that the .30-06 had a fair amount of empty space in it when loaded with similar powders to the 7.62 X 51. Short and fat helps but once you have similar loading densities it's importance goes down and the advantage only shows up in guns/ammo that are shooting at under 1 minute of angle anyway.
Design a cartridge that will work in -40 degrees and also work at 130 degrees F.

 

[Ivan1GFP]

Hello Shortround6,
Obviously you have done quite a lot of thinking about the different aspects of an ideal rifle cartridge.

For use in full auto you need to reduce recoil somewhat, how much is subject to argument but since recoil is proportional to momentum (weight/mass X velocity) and not energy you have two choices, cut bullet weight or cut velocity (or both). Keeping 140-150 grain Spitzers (with or without boat tails) doesn't cut the recoil enough to give you a controllable weapon in full auto. Please remember that the US M2 Ball .30-06 used a 150 grain bullet and the Nato 7.62 used a 147-150 grain bullet. Nobody made a controllable full auto rifle in either cartridge. I don't know if cutting recoil by 20-25% is enough but it is a much better start than cutting velocity by only 10% or so.

What you are stating is pretty much conventional wisdom on the subject of recoil.
I believe there are a couple other factors to consider.
First of all, using the .30-06 M2 Ball as an example, what comes out the muzzle is a 152 grain bullet at 2800 fps but there is also around 50 grains of solid propellant that is also converted to gas and sent out the muzzle at a speed even higher than the bullet.
If we have a lesser amount of propellant, there is less mass being sent out the muzzle.
Next, consider that the typical battle rifle of the time either did not have a muzzle brake or did not have an efficient muzzle brake.
I have experienced what a really good muzzle brake can accomplish.
A fellow brought a .50 caliber BMG bolt action (McMillan) rifle to the range one day and wanted to know how fast his handloads were.
Since I had a chronograph in operation, he asked me to fire a few rounds for him. The idea was that if I blew up my own chronograph, he was not to blame. His 650 grain bullets moving at around 2650 fps felt about the same in recoil as the 7,92mm Mauser I had at the range. The muzzle blast was horrendous, but the recoil was not painful. Of course his rifle was fairly heavy but I believe it was because of a very efficient muzzle brake he had on the gun which make it look like a miniature anti-tank gun.
I believe that a proper muzzle brake can do a lot to reduce felt recoil.

In 6.5mm a 120 grain bullet will give (given the same shape) as good or better ballistics than a 150-168 grain .30 cal bullet. A 140 grain 6.5mm bullet acts like a 190-200 grain .30 cal.
You don't need the heavy 6.5mm bullets (which also require quicker twist rifling) unless you are shooting to distances beyond the normal employment ranges of a bipod mounted machine gun.

Part of the reason for keeping a heavier bullet is to keep the hitting power fairly high even at long range. A 120 grain bullet weighs less and must make up for it with a higher velocity. On a paper target, it does not matter but that isn't the kind of target that we are going for.

Besides, what is the disadvantage for a quicker rifling twist?
I have had bullets come apart in 1 in 7 inch twist .223, but those were very flimsy bullets.
In general, when barrel quality is questionable as it would be in a mass production military rifle, faster rifling twist is better than slower.
To be honest, I am not that set on a super heavy 6.5 mm bullet in the standard rifle cartridge. My idea is that such an option should be available for long range use and the rifle should be capable of using it.

Please note that the Swedish Mauser uses anything from a 120 grain up to about a 160 grain bullet with a rifling twist that is around 1 turn in 8 inch or 8.5 inch.

The US adopted the .30 M1 with a 170-something grain bullet for use in water cooled 1917 Brownings where it increased the max range by something like 1500-2000 yds. Effective range was increased but not by quite as much. Please note this was for indirect fire before 81mm mortars became standard issue.

Our Ideal "rifle" for squad use has no such need for that kind of capability.

in .30 Caliber, the 173 grain match bullet seems to behave reasonably even when going through the trans sonic region.
The typical 168 grain HPBT match bullet does not behave so well and becomes unstable as it drops through trans sonic.
For a 168 grain bullet fired from a .308 Winchester at about 2600-2650 fps, it goes trans sonic just beyond 900 yards IIRC.
That was the main reason for the creating of the 155 grain Palma bullet. It needed to remain accurate beyond 1000 yards so needed to be pushed faster at the muzzle. (There is a lot more to the story but the point is that the ballistic coefficients we are seeing with these bullets are no guarantee that they are accurate to the ranges needed by a machine gun.

I am not too sure how well sharp shoulders really feed in automatic weapons. In any case any advantages they have are mostly theoretical and usually will be unnoticed in a military grade rifle. After I had my 6.5 X .308 for a few years (bolt action) a fellow who shot on some of the teams I did had a rifle built in 6.5mm X .308 Ackley Improved (blown out, sharp shoulder) and he gave me an empty case. Weighing his case and mine empty and full of water to the top of the case mouth his case held slightly more than 1 grain of water more, out of just over 60 grains. Seating a bullet cuts in the capacity quite a bit, but a 2-3% increase in powder capacity doesn't really mean much and a military weapon is much better served with a slightly lower pressure level than going for that last 2% in performance. Short and fat has gotten some good results in bench rest shooting but please remember that the .30-06 had a fair amount of empty space in it when loaded with similar powders to the 7.62 X 51. Short and fat helps but once you have similar loading densities it's importance goes down and the advantage only shows up in guns/ammo that are shooting at under 1 minute of angle anyway.
Design a cartridge that will work in -40 degrees and also work at 130 degrees F.

Sharp shoulder is relative. I was thinking of something at about 30 degrees as in the 6 mm PPC cartridge. Even 25 degrees would do and that beats the 17.5 degrees of the .30-06.
The .308 Winchester by reputation does not seem to gain much when Ackley Improved and I suppose the offspring based upon it would not gain much either. As you pointed out, the shape really does not change by much.
Parker Ackley's "Improved" chambers were a pretty good idea back in the day when the typical cartridge case had a very tapered body with only a slight shoulder angle. Lots of folks "improved" chambers by reaming them larger but Ackley's were special in that the improved chamber was still capable of safely firing the factor "non-improved" ammunition.
(I am sure you know this but some folks here probably don't.)
You are absolutely correct that the .30-06 has a lot of empty space in typical military loadings and I believe that is not optimal and has been stated as one of the reasons why the .308W is inherently more accurate.
There does need to be enough capacity to allow flexibility in bullet weights and different compositions of powder.
The idea of a sharper shoulder case with very little taper is to avoid designs like the 7.62 mm x 39.

- Ivan.