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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; 1903 views.
gatnerd

From: gatnerd

21-Nov

The XM915 lightweight 20mm gatling designed for the new FARA light helicopter has now been debuted:

https://www.thedrive.com/the-war-zone/43215/this-is-our-first-look-at-the-armys-new-20mm-aerial-cannon-on-an-actual-helicopter

XM915 20mm firing

US Army video

Specs are 115lb, 20x102mm, firing either 750 or 1500rpm. 

This is surprising in that the earlier XM301, also by GD and designed for the Commanche helicopter, was a much more svelte 80.5lbs.

All in all 20mm seems an odd choice vs the 130lb M230 in 30x113mm, given that this gun is not that much lighter, yet offers far worse explosive effects.  

stancrist

From: stancrist

21-Nov

gatnerd said:

20mm seems an odd choice vs the 130lb M230 in 30x113mm, given that this gun is not that much lighter, yet offers far worse explosive effects.

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

roguetechie

From: roguetechie

21-Nov

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.

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

Mr. T (MrT4)

From: Mr. T (MrT4)

21-Nov

20mm over 30 is likely down to multiple factors , light weight FARA airframe limitations in terms of payload and recoil, but it might also be in regards how its employed ,FARA being light scout is basically upgrade to OH-6 and OH-58 with their 7.62 miniguns. Its not the primary weapon, more of a backup sidearm.

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

Number of WW2 era aircraft canon in 20-23mm  fired at roughly 700rpm and most weighed under 40kg some under 30kg

Widespread  GSH-23 is at cca 50kg with much higher cyclic rate

Czech derivative ZPL-20 compatible with the same 20mm ammo is at 63kg and features 750-2500rpm cyclic rate.

stancrist

From: stancrist

21-Nov

"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

stancrist

From: stancrist

21-Nov

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. 

gatnerd

From: gatnerd

22-Nov

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. 

stancrist

From: stancrist

22-Nov

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.

roguetechie

From: roguetechie

22-Nov

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.

gatnerd

From: gatnerd

23-Nov

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
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