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.