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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons, particularly in larger calibres (12.7+mm).

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Aluminum Cases   General Military Discussion

Started 5-Aug by JesseH1234; 2023 views.
renatohm

From: renatohm

5-Aug

Briefly, aluminum is very reactive, and a shot generates enormous pressure and temperature, which may generate a thermite reaction.

https://en.m.wikipedia.org/wiki/Thermite

If that happens, the aluminum will burn through bolt, barrel and whatever else is in the way.

Several attempts have been made to use aluminum as case material because it's density (2.7 grams / cubic centimeter) is much lower than steel (usually between 7.75 and 8.05 g/cc) and brass (8.4 to 8.73 g/cc).

But it's very hard to find a solution for high performance ammunition (read rifles and bigger). Some successes have been made with handgun ammunition, but even then the chemical reactions prevent the reloading of aluminum cases.

One solution is a plastic liner, which has the side effect of lowering the internal volume and/or increasing pressure.

As far as I know, there have been only two successful uses of aluminum cases for big guns - for the 30x173 of the A-10 (saves nearly a ton) and for the 30x113B of the AH-64 Apache (don't know how much weight is saved, but surely is a lot).

stancrist

From: stancrist

6-Aug

renatohm said:

aluminum is very reactive, and a shot generates enormous pressure and temperature, which may generate a thermite reaction.  If that happens, the aluminum will burn through bolt, barrel and whatever else is in the way.

Not going to happen.  Solid aluminum is not thermite.

During the Vietnam War, countless numbers of M113 APCs were hit by RPG rockets and recoilless rifle rounds, making finger-sized holes in the vehicles' aluminum hulls, but they did not ignite the metal.

Shaped Charge Warhead GIF | Gfycat

Bar-Armor Experiments on M113 APCs in The Vietnam War - Tanks ...

autogun

From: autogun

6-Aug

I have long been surprised that the various attempts to make aluminium-cased 20 x 102 ammo haven't succeeded in getting into service. After all, aircraft systems are weight-critical, and most M61 installations have come with a large quantity of ammunition, so the weight saving should be considerable. 

I can only guess that making aluminium cases is considerably more expensive than brass or steel because of the care needed to to ensure that the fireproof case inner liner is intact. 

Note that when 30 x 173 ammo started to be used in AFV guns the case material reverted to steel very quickly.

jxexqx

From: jxexqx

6-Aug

Aluminium is quite "grippy"unlike brass and that might also lead to extraction troubles? Again, a coating would help but it all adds to the cost.

JesseH1234

From: JesseH1234

6-Aug

Well I don't have a full list, but I did look up the pressure on what I believe was the Navy 20mm round, and it was over 60,000psi chamber pressure.   So perhaps there is a threshold there, especially for a round not designed to use aluminum to begin with.  We know it works fine with handgun ammo, although specifically to what pressure I cannot find (has to be 30k though I think). 

Without the dimension limitation based on other materials though, one could always create enough strength using thicker walls; aluminum is stronger for its weight than brass or steel.  Grippiness is true, although it depends against which substance....and that could be mitigated with case taper. 

Anyway I just wondered if there was some very specific material limitation anyone could point to.  It seems it would be far easier to solve any problems with an aluminum case than a polymer one, especially given that depending on alloy, aluminum can handle wide temperature swings better than most metals and certainly most polymers. 

autogun

From: autogun

7-Aug

I'm wondering whether the problem lies in the technology for fireproofing the cases. Whatever kind of lining is used, it has to be 100% intact or the consequences could be serious. If you have a 30 mm wide case neck to work through, that might be easier than a 20 mm one, in terms of applying the lining and checking that's it's intact.

Chamber pressure of the GAU-8/A ammo is 423 MPa which is over 61,000 psi, so that's no problem. 

stancrist

From: stancrist

7-Aug

AN ANALYSIS OF 5.56MM ALUMINUM CARTRIDGE CASE BURN-THROUGH PHENOMENON

https://pdfs.semanticscholar.org/a8ef/28a9bfd715829742ad7c3b25c11012558b6a.pdf

========================================================================

A Critical Assessment of the Aluminum Cartridge Case Failure Mechanism

https://apps.dtic.mil/dtic/tr/fulltext/u2/a028269.pdf

EmericD

From: EmericD

7-Aug

Well, those 2 papers clearly states that even if "solid aluminium is not thermite", during "burn thru" failure there is an exothermic reaction between the hot gaseous products and the aluminium case, resulting in additional damage compared with a simple "gas leak" (with brass case).

So, yes, during a "burn thru", part of the aluminium case is actually burning...

stancrist

From: stancrist

7-Aug

The fact that "part of the aluminium case is actually burning" is quite different than what my comment addressed.

Renatohm said that burn thru "may generate a thermite reaction...the aluminum will burn through bolt, barrel and whatever else is in the way."

You don't seriously think that burn thru would make the aluminum cartridge case act like a miniature thermite grenade, do you?

EmericD

From: EmericD

7-Aug

stancrist said:

You don't seriously think that burn thru would make the aluminum cartridge case act like a miniature thermite grenade, do you?

No, because in a thermite grenade it's the liquid iron produced during the reaction that is making most of the destructive work, but as shown in the paper you linked, the gun erosion produced by the "burn thru" mechanism is much more severe than damage produced by a gas leakage with conventional brass case, because aluminium burning is exothermic.

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