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. 

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

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...

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?

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.

EmericD said:

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.

Nevertheless, solid aluminum is not thermite, and does not burn like thermite.

Thermite is powdered aluminum mixed with a powdered oxidizer.  When thermite is ignited, it will continue to burn until completely consumed.

The cartridge case is solid aluminum, and the only available oxidizer is in the propellant.  Once the propellant is consumed -- which occurs in a fraction of a second -- the aluminum case stops burning.

It is simply not possible for an aluminum cartridge case to "burn through bolt, barrel and whatever else is in the way."  There can be no "thermite reaction" when there is no thermite.

stancrist said:

Once the propellant is consumed -- which occurs in a fraction of a second -- the aluminum case stops burning.

Not totally according to the papers you linked, as Al reacts mainly with H2O, which is a byproduct of the primary reaction (so even when the reaction stops, H2O remains), to produce aluminium oxide and hydrogen, that could later mix with air to burn again (hence the large and bright plume on the various pictures as shown in the report).

But I agree that the aluminium case will not "burn through bolt & barrel", it will just severely erode those parts and render the weapon inapt for service, and try to harm the soldier in the process.

Just for the record, during the peak of the "FAMAS crisis" in 2008-2009, we had 1 out-of-battery explosion for ~500,000 cartridges fired, and it was ten times more than what is OK for a front line service weapon, and the report is talking about 1 "burn-thru" out of 25,000 rounds with aluminium cases, so even with a reliability increase of a factor 20 the Al case would still be a major problem...

The issue is actually that ACs don't fire anywhere close to as many rounds as small arms, and when they have problems no one's nearby. If 1 in a million autocannon rounds burns through, you might lose a barrel every 100,000 sorties. If the same rate of SAA fails, that's 1,000 injuries and close calls every year.

EmericD said:

stancrist said: Once the propellant is consumed -- which occurs in a fraction of a second -- the aluminum case stops burning.

Not totally according to the papers you linked, as Al reacts mainly with H2O, which is a byproduct of the primary reaction (so even when the reaction stops, H2O remains), to produce aluminium oxide and hydrogen, that could later mix with air to burn again...

Picky, picky.  The aluminum stops burning a fraction of a second after the propellant is consumed.  It doesn't keep burning until the entire cartridge case is consumed, unlike thermite, which burns until there is none left.

EmericD said:

Just for the record, during the peak of the "FAMAS crisis" in 2008-2009, we had 1 out-of-battery explosion for ~500,000 cartridges fired, and it was ten times more than what is OK for a front line service weapon, and the report is talking about 1 "burn-thru" out of 25,000 rounds with aluminium cases, so even with a reliability increase of a factor 20 the Al case would still be a major problem...

Certainly.  Which is no doubt a big reason why aluminum cases have not been adopted for rifle ammo.

Picky, picky.  The aluminum stops burning a fraction of a second after the propellant is consumed.  It doesn't keep burning until the entire cartridge case is consumed, unlike thermite, which burns until there is none left.

Ah, OK, but it's just because the propellant does not produce enough H2O to burn a significant part of the case (the oxygen balance of most powders is made to avoid complete oxidation of the final products, hence the secondary flame found on most weapons).

Yes, in the case of a thermite, the balance of oxidizer and fuel is properly done to achieve a complete reaction, and as I said previously, in the case of a thermite it's the liquid slag (mostly composed of iron) that is doing most of the demolition work, we don't have this liquid slag in the case of a "burn thru".