Military Guns and Ammunition

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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.

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XM915 20mm debuted    Aircraft Guns

Started 21-Nov by gatnerd; 5558 views.

From: stancrist


"Engineers have found that a 20mm Gatling weapon provides sufficient range and lethality while minimizing system weight and forces applied to the FARA airframe."

Future recon aircraft competition moves forward featuring a 20mm Gatling gun developed at Picatinny | Article | The United States Army


From: stancrist


roguetechie said:

There's also the fact that our helicopters use their guns less and less as the years go by.

And the changing nature of gun targets we engage.

IIRC, the Army is concerned with suppression/destruction of targets expected to be encountered in near-peer combat, primarily MANPADS and SAM systems that would pose a danger to FARA and  follow-on attack aircraft.

I don't know whether 20mm or 30mm would be more effective for that purpose.

roguetechie said:

More or less, of the things we can still use helicopter guns to engage a 20mm is going to work just as well while letting you carry at least a few more rounds of 20.

I don't necessarily see this as a bad thing

Well, OTBE, I think more ammo is always better.  sunglasses

But, I'm not sure if 20mm would necessarily let FARA carry more rounds than 30mm would. 


From: gatnerd


stancrist said:

Perhaps 20mm has lower recoil forces than 30mm, therefore better suited to a lightweight recon helicopter?

Lower recoil forces would be a valid reason, although trying to develop a 'soft recoil' 30x113mm would seem to be worth pursuing if that's the case.

Otherwise, going 20mm seems pretty dubious. 30mm has an aluminum case vs brass for the 20mm, so ends up being fairly close to each other in weight, while having a much more useful HEDP projectile, as well as the new proximity fuse function. When the similar gun weights are factored in, 20mm is an odd choice, outside of the recoil angle. 

30mm would also have ammo commonality with the armys Apache, and soon to be ground base JLTV's. 


From: stancrist


I'm inclined to agree with you on nearly all of your 30mm > 20mm points.

Regarding weight, 500 rounds of 20mm is 100 lbs lighter than for 30mm.

That would have some effect on flight performance.  How much, I dunno.


From: roguetechie


The obvious answer here is neckless true velocity 20mm...

Though I do also like your idea for a "soft recoil" 30x113 which is very doable.


From: gatnerd


stancrist said:

Regarding weight, 500 rounds of 20mm is 100 lbs lighter than for 30mm.

Thanks, I hadn't had the exact weight.

Whats interesting is that the Apache originally started off with a 1200rdx30mm capacity, but now only carries ~300rd of 30mm (reportedly due to the addition of an internal fuel tank). With the decent accuracy plus decent bang of the 30mm, this has proven adequate so far. And at least for Anti Personnel, we can assume the addition of the new proximity airburst rounds will make that much more effective use of those 300rds.

In terms of weight, how do these stack up? 

115lb XM915 + 500rd 20mm 

130lb M230 + 300rd 30mm 

In terms of relative effectiveness, one simple method we can use is the lethal fragmentation range for the two shells x # of rounds carried to get a sort of 'area of fragments' rating for each loadout. 

20mm reportedly has a 2m lethal radius. 2m radius results in 12.57 square meters of fragmentory lethal area per shell. x500 is 6,285 m2 of fragment coverage 

30mm reportedly has a 4m lethal radius. 4m radius = 50.27 m2 of fragmentory lethal area per shell. x300 = 15,081 m2 of fragment coverage.

No doubt this is an imperfect analysis, but does give an idea of just how much more effective 30mm is. Even with 40% fewer shells, it has 240% more fragmentory area. 

And even here, I think this still oversells the 20mm's effectiveness. While 2m radius is claimed, I suspect actual effectiveness is closer to 1m:

-20mm is designed for anti aircraft/ anti material work, and so produces a a few handfulls of large fragments designed to tear through machines, not thousands of small fragments like 30mm which is designed for anti personel. The fewer frags, the worse hit probability becomes as the radius expands.

-20mm, being anti material, is designed for impact / impact delay function, and when fired air to ground into sand/dirt/mud, the round will almost certainly burry itself a bit into the ground before detonation (this same problem plagued 30mm and drove the development of Proximity fuse.)

If the (in my eye, more realistic) 1m radius is used for 20mm, we get 3.14m2 per shell, or 1,570m2 per 500rd, vs the 15,081m2 per 300rd of the 30mm. That essentially a 10x increase in fragmentation coverage.  

  • Edited 23 November 2021 1:04  by  gatnerd

From: gatnerd


Mr. T (MrT4) said:

For me the surprising part is going for a Gatling vs a revolver canon when you only have 750 and 1500rd  cyclic rates.

That is a good point; pretty low rpm for a Gatling. Plus I believe each 'burst' with Gatling ends up ejecting a few live rounds as well as the gun slows down. 

I know Northrop is trying to offer their 20mm SkyViper chan gun as an alternative, which they claim will have a higher ROF then the M230's ~625rpm

“The M230 on the Apache started out with a 1200-round magazine initially. With significant advances in the accuracy of the weapon and fire control system, they’ve reduced [the magazine] to just a few hundred rounds.”
“Our view,” Canole says, “is that, at the the end of the day, a significantly more accurate weapon pays greater dividends than one that fires more rounds at a higher dispersion.”
Northrop Grumman declined to offer Sky Viper’s rate-of-fire or weight details but Canole says it will match the XM301’s weight while offering a greater firing rate than the current M230.
The company says its configuration is more reliable and easier to maintain than a Gatling-style gun with multiple rotating barrels. The reduced recoil of the Sky Viper - which Northrop Grumman says is “ground-up” redesign of the M230 - diminishes airframe fatigue and gimble/mounting structure stress, rendering lighter system weight as a whole.

From: stancrist


Here are the weights I used.  (From TM43-0001-27  Army Ammunition Data Sheets)

   20mm  M56A3   HEI     3965 gr

   30mm   M789   HEDP   5371 gr


I think your "area of fragments" comparison is badly flawed, because it assumes that (a) the entire ammo load will be fired, and (b) there would be an even distribution of projectiles and their fragments.

More importantly, I think you are placing far too much attention on anti-personnel effectiveness, when the Army is concerned about engaging targets like this:

Which is better for hitting such a target as fast and as hard as possible:

   1500 rds/min @ 3400 ft/sec?


     600 rds/min @ 2600 ft/sec?


From: gatnerd


stancrist said: I think your "area of fragments" comparison is badly flawed, because it assumes that (a) the entire ammo load will be fired, and (b) there would be an even distribution of projectiles and their fragments

Which is better for hitting such a target as fast and as hard as possible

Its not so much firing the whole load, but rather showing area per loadout. I'd think the #'s would hold up the same for comparing say 25rd bursts between weapons; basically the point is that for any given burst size, the 30mm will be able to have a much higher area of fragments / greater fragment density per area. 

Now in terms of destroying vehicles, 30mm is rated to defeat 25mm RHA at 50 degrees using its HEDP design. I've not been able to find an exact penetration level for the 20mm (Nammo says over 10mm at 1000yd) but I'd be surprised if the HEI/SAPHEI was equal/better then the 25mm at 50 degrees of the 30mm HEDP. 

ROF and time of flight is better for 20mm though, which could be important for hitting a target while flying. 


From: renatohm


One transonic round every few seconds, far away from flak's effective range.

Seriously now, if they're thinking about dealing with air defenses better than AKs (or the occasional MANPADS / RPG wielder), then neither 20mm nor 30mm won't do it.

If they do want to use it for spray and pray against scattered soldiers, either will do, but I'm also of the '30mm is better' team.