German 15mm autocannon

[Shortround6]

Effective in what way?

15mm simply describes the diameter of the projectile. Not it's weight or velocity. The German 15 X 96mm round (15mm dia projectile and 96mm case length) was rather high velocity but while bigger than a 12.7mm/.50 cal bullet it was quite a bit lighter than a 20mm Projectile.

From Tony Williams most excellent website. BOOKS BY ANTHONY G WILLIAMS

The AP projectile went about 72 grams at 850 m/s but the HE only weighed about 57 grams compared to most 20mm HE projectiles going about 95-130 grams with HE content to match.

Against unprotected troops a solid bullet is pretty much a solid bullet (dead is dead), against trucks, parked aircraft, small boats (small meaning fishing trawlers, motor torpedo boats and landing craft), trains and even small buildings the 20mm HE is going to more effective. Against armored vehicles the 15mm may be more effective against light armor ( light tanks, armored cars, APCs and SP artillery.) 25-30 ton tanks are NOT going to be taken out by 15-20mm aircraft cannon except by lucky hits.
The 15mm AP round is more effective than some 20mm ammo and less effective than others.
15-20mm AP ammo isn't really much good against concrete bunkers or even heavy old stone buildings.

 

[Frantish]

Effective in what way?

15mm simply describes the diameter of the projectile. Not it's weight or velocity. The German 15 X 96mm round (15mm dia projectile and 96mm case length) was rather high velocity but while bigger than a 12.7mm/.50 cal bullet it was quite a bit lighter than a 20mm Projectile.

From Tony Williams most excellent website. BOOKS BY ANTHONY G WILLIAMS

The AP projectile went about 72 grams at 850 m/s but the HE only weighed about 57 grams compared to most 20mm HE projectiles going about 95-130 grams with HE content to match.

Against unprotected troops a solid bullet is pretty much a solid bullet (dead is dead), against trucks, parked aircraft, small boats (small meaning fishing trawlers, motor torpedo boats and landing craft), trains and even small buildings the 20mm HE is going to more effective. Against armored vehicles the 15mm may be more effective against light armor ( light tanks, armored cars, APCs and SP artillery.) 25-30 ton tanks are NOT going to be taken out by 15-20mm aircraft cannon except by lucky hits.
The 15mm AP round is more effective than some 20mm ammo and less effective than others.
15-20mm AP ammo isn't really much good against concrete bunkers or even heavy old stone buildings.

So, Yes, a 15mm AP can be more effective that 20mm HE in destroying lightly armored and soft targets.

 

[Shortround6]

Against "soft" targets like trucks the 20mm He may be more effective. Depends on where the hit is, a 15mm hole in a canvas tarp or wood floor board is hardly going to put the truck out of use. granted a 20mm hole in the tarp won't either but a 20mm shell exploding in the floor baord is going spray around some fragments and splinters. A 20mm hit in the fuel tank could open the fuel tank like a giant can opener rather than punch a hole in it. The 20mm can make a much larger hole in the side of a boat.

The 15mm AP falls into a very narrow useful band. The lightly armoured target has to have enough armor that a 7.62-8mm bullet (steel cored) won't penetrate and perhaps a 12.7mm won't penetrate yet the 15mm will. Yet the armor cannot be so thick that that 15mm can't penetrate penetrate while 20mm and larger ammo will?

Armor penetration is about how much force you can apply to how big an area ( diameter of the projectile) and force is is mass X velocity squared. Projectile weight, as a general trend, tends to go up with the cube of the projectile diameter. A 20mm using a 130 gram projectile has about ( with a few percent) of the same mass per unit of target area as a 72gram 15mm projectile so penetration is pretty much dependent on striking velocity.

For the Germans the 15mm gun weighed, took up as much volume and cost the same as a 20mm gun and was not as useful against as wide a variety of targets.

 

[KillaKiwi]

I think it actually makes a lot of sense to reintroduce the 15mm MG 151 from a German point of view.

German aircraft armament and ammunition was always very specialized for a specific role and that role was to be effective against planes, either fighter or bomber.

In 1941 the 15mm MG 151 didn't have the punch to deal with fighter planes in the same way a 20mm could with Mineshells but in 1944 even several 20mm cannons were not that effective in taking out bombers.

There were two ways to deal with bombers and Germany focused most of it's attention to the later option. Destroying the bombers frame with large, high capacity explosive shells in as few hits as possible.

Neither 30mm Mk 103 or Mk 108 was particular great as fighter armament due to either weight, recoil or muzzle velocity and they still needed several hits to take out a four engined bomber. 55mm cannons and rockets carrying around 0.5kg of explosive were developed to reduce that number to a single hit.

However, incendiary ammunition also turned out to be quite effective against bombers (well any plane) and it was realized that incendiary ammunition in general was a lot more effective than explosive ammunition for low calibers guns. Both MG 131 and MG 151 had their explosive ammunition discontinued in favour for pure incendiary shell.

US tests with 12.7mm, 15mm and 20mm ammunition showed that 15mm Incendiary shells had a very high likelyhood to ignite a fuel tank, pretty much on par with 20mm incendiary shells.

Now, for the US it wouldn't make sense to actually field a 15mm cannon, since they already had 20mm cannons which were effective against fighter planes but with Germany requiring weapons that would be primarely effective against bombers it doesn't seem so far fetched to replace the MG 151/20 for the MG 151 to allow fighters to spray bombers from long range with high velocity 15mm incendiary rounds.

While the MG 151/20 was quite effective against fighter planes for its time, it lacks the velocity to engage bombers from range.

Of course the 20mm MG 213, featuring a more powerfull cartridge allowing for much higher muzzle velocity would have been able to combine both aspects.

 

[spiralcopter]

The MG 151 was a good enough weapon that the US had a number of projects attempting to clone it. I believe the T17 .60 caliber is the best known, but there were actually a number of parallel projects to make either production-streamlined clones of the MG 151 or entirely new weapons based around the .60 caliber ammo, which had very similar ballistics to the 15mm round of the German weapon.

In terms of it being an autocannon vice a very heavy machine gun, the Germans certainly considered it a machine gun, but the ammunition had a driving band on the projectile, which is another dividing line that some use to differentiate MGs vs autocannons. The point is academic, but it's important not to let the terminology trick you into thinking that because the MG 151 and M2 were both heavy machine guns they were in the same class. The 15mm round is about 60% more powerful than the US 12.7mm round.

Changing the weapon from a (very) heavy machine gun into a cannon principally involved necking out the case and installing a new barrel. There were a few other changes, but the installation really wasn't much different (in terms of the weight and volumes needed to accommodate the ammunition and the weapon itself). The Soviets would take this concept one step further and develop a family of autocannons where the caliber of the weapon could be changed with literally just a barrel swap.