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Without further information everything is speculation.
Unless there is a typo. Germans used the 15 X 96 cartridge without belt.
Using only a 69mm cartridge length does not give enough powder capacity unless there is a lot of bottle neck (very fat case).
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If you want the kinetic energy of the US .50 cal you have generate the energy by using a similar amount of propellent.
Maybe not identical but you don't get something for nothing. Using a smaller case capacity and heavier bullet/projectile can get a bit better efficiency
but trying to drive 57 Gram projectiles at 750mps is not going to be easy.
The 12.7 X 81 ammo drove 33-35grams bullets at about 750mps. Driving bullets that are about 60% heavier at the same speed is going to call for around 60% more propellent.
A pictures/drawings of the 15 X 69B cartridge?
Edit.
Propellent weights for a few cartridges.
13 X 64 = 7.1 grams
12.7 X 81 = 9.2 grams
12.7 X 99 = 15.6 grams
15 X 96 = 24.5 grams.
It is very surprising why German never put it into use?
Were the 15*69B cartridge a better option for early WWII fighters
things do not scale well. If you increase caliber the general rule of thumb is to cube things.Having a belt-fed heavy MG (that is perhaps 60% the size and weight of the MG 151/15) improves their overall firepower. Especially if the heavy MG can fire shells, small as they are.
Now a Japanese Ho-103 gun was about 22kg or about 3kg lighter than a 60% MG 151.
A lot depends on the cartridge. 12.7 X 81 necked up to 15mm and the gun lengthened a bit hold the shell?
Use the 15 x 69b case several years early to keep the overall cartridge length down?
Is the short case going to do what you want with the propellants of 1939/40 or do you need 1944 propellants and pressures ?
You might be able to build a Ho-103 equivalent using a short 15mm round but I think the velocity is going to be a lot closer to 600m/s than to 750M/S.
Thing was that perfect weapons were too late, and many of belligerents started the war with weapons that were either woo weak (sometimes and somewhat countered by installing a lot of them, but also many times it was one or two pairs of LMG on a fighter), or with too slow MV, or with low RoF, or were unreliable, or a fighter might fit just one of them. Or a combination.I'm sure all of the WWII combatants would have loved to have a high velocity, high rpm, reliable, and decently lightweight autocannon from the get-go. But alas, for various reasons it wasn't to be.
Thing was that perfect weapons were too late, and many of belligerents started the war with weapons that were either woo weak (sometimes and somewhat countered by installing a lot of them, but also many times it was one or two pairs of LMG on a fighter), or with too slow MV, or with low RoF, or were unreliable, or a fighter might fit just one of them. Or a combination.
Back to the Germans, having a powerful enough HMG, that is belt-fed, with decent MV and RoF (the OP is realistic here with suggesting moderate MV and RoF) would've been a benefit when looking at what they fielded in the 1st 2-3 war years on their A/C - fighters, bombers and attackers.
Perhaps the best was the Shvak. It didn't tick the 'has a very powerful shell' box, but that was probably the only shortcoming it had.es, that was what I was hinting at with my previous comment. Reasons varied, but the bottom line being that nobody really entered WWII with a good autocannon, even though many had recognized the advantages of it.
Probably yes. Did the Germans have some similar thinking as the British, in that they were planning to jump straight from their LMG to autocannons? Seems their Mg 131 only came later as a way to boost the punch of the cowling guns already after the LW had adopted autocannons?
A far better weapons' layout than what the Emils carried. Would've also been a better weapon than what the bombers carried.So what if the Mg 131 would have been available earlier? Would we have seen Bf 109E's armed with 4 Mg 131 (two in the cowling plus two in the wings) instead of the historical 2 LMG's + 2 MG-FF's in the wings? And would the 131 have become their 'standard' defensive gun for bombers, similar to the M2 used by the Americans bombers?
One can quibble that the round used by the Mg 131 was too weak compared to other HMG class rounds, but I'm not so sure. It compensated by being a lightweight gun, high rpm and as a result of the modest muzzle velocity, good barrel life. Perhaps the 12.7x81SR round used by the Italian Breda-SAFAT HMG would have been a better choice, with a bit more MV, without going all the way to a really high power cartridge like the .50 BMG?
Or how about full sized Ho-5, with being necked down to 15mm, for being 15x94?or, sorta 3/4 of a Ho-5
Limits of the platform still apply - ie. can you fit it easy within the wings of the Bf 109E, as well as a defence gun positions on the bombers?Or how about full sized Ho-5, with being necked down to 15mm, for being 15x94?
It does make for a heavy 85lbs gun, 104 with charging and feed accessories, but reliable gun with an overall 64" length, if keeping with the 750-850rpm.
For the cartridge case, likely to be higher MV close to the 15mm MG 151 of around 900
That's better than the .55 Boys ATR in performance, at 20 lbs more weight than a regular M3 50 aircraft mount.
Well, the Japanese used the 72 pound Ho-1 20mm with a 15 round magazine as a flexible gun in bombersLimits of the platform still apply - ie. can you fit it easy within the wings of the Bf 109E, as well as a defence gun positions on the bombers?
The ammo it uses is of the too low power, much better to use the Belgian/French 13mm round as a starting point. Even the USor Soviet12.7mm is better bet. 13mm TuF, too.
Same things, for better or worse.or, Browning HMG, with barrel for 15mm ammo
If those could fit in the Italian FIAT G55,
could in the -109, provided the Germans were willing to use blast tubes, where they were not in OTL moving from the FF, going to the 151/20 underwing, from the ammo setup
rejigger the wing, have the Ho-5 close to where the FF was, and have a tray for the belt.
Same things, for better or worse.
Same rim area and back of cartridge, with adjustments to the neck to suit the 13 (13.2)mm bullets. But that means just about the same overall length of cartridge and length of bolt travel.
Probably slightly longer if you want a 15mm projectile unless you use a short stubby 15mm shell.
Now in 1938-40 there may be a propellent problem. French, Belgians and Japanese got around 790-800ms using slightly heavier bullets than the US .50 used, but actually a big faster than the older .50 cal ammo.
It does mean that you are stuck with the weight of the .50 cal Browning and please be sure to include all the bits and pieces which the 29kg weight of the .50 sometimes listed does not do.
750 m/s should be enough for the 1st war years. Keeps the weight and size of both guns and ammo within modest limits.The German 15 X 96mm round used 57% propellent than the US .50 cal did. You can neck the .50 cal up to 15mm fairly easily. US tried at least a 16mm version if not others.
The question is what kind of velocity can you get out of it. General rule of thumb is you need about 20-21% more powder to get 10% increase in velocity (the increase it kinetic energy has to come from somewhere)
It seems to have taken around 6 years for most countries to get a gun from start up to service use. The MG 131 was mentioned in some 1939 specifications. It was used in 1941 in some bombers, it did not get into fighters until 1942. Not sure if there was an ammo problem. There may have been both mechanical priming (firing pin) and electric priming.
Mounting some of these guns was a little complicated too.
US .50 cal M2 used more propellent per round than the 20mm MGFF/M did. and fired about 50% faster. Granted it used lighter projectiles but you can't use the same mounts/brackets you use for 7.7-7.9mm machine guns.
G.55 was with the bigger and thicker wing, so there is no wonder that MG 151 was a reasonably easy fit.
Germans themselves were not shy to add bumps when the weapon installation required so, already staring with the 109E-3*. Bumps don't solve the structural issiues, however, that was the reason why the Spanish added another spar so the wing does not break due to the weakened main spar. See here, for example.Seems it didn't need all that thickness for the receiver and belt tray, a little bump on the top wouldn't be the end of the world, and would be smaller than with what the Spitfire ended up with
Seems like the Germans didn't do enough with the Wing in going from the E to the F and G models.Bumps don't solve the structural issiues, however, that was the reason why the Spanish added another spar so the wing does not break due to the weakened main spar. See here, for example.
750 m/s should be enough for the 1st war years. Keeps the weight and size of both guns and ammo within modest limits.
And we are back to the conflicting basic requirements. With faster aircraft as targets (and faster shooting platforms) you have less time to aim and less time that the target is within the firing area/impact zone. Do you worry about getting hits or the effect of the hits first?I agree, and further I think one can argue it would be close to the sweet spot also later in the war, as cannons relied more on the effects of the shell exploding rather than kinetic energy.