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A proof mark is applied when a barrel is tested with a "proof" charge that generates more pressure than the maximum for the cartridge. The loads are sometimes called "blue pills". Don't know why because U.S. proof loads are usually in nickel cases and have a red lacquer paint on the base.

One U.S. maker agrees with SAAMI to load them and they are sold only to manufacturers. There is also "reference" ammo used to calibrate pressure test equipment. The tester then stamps a "P" on the barrel or sometimes "VP" for verified proof

A cartouche is a mark applied to identify the maker or unit
 

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My understanding is that the cartouche is an inspection mark, not a proof mark.

The US has no "proof house", so a "proof" mark only means what that particular manufacturer says it means. Also, with US arms "proof" isn't nearly as technical as European "proof". Most US arms, both military and commercial are subjected to the "blue pill" load and then the weapon is visually inspected after firing. In some cases it is magnafluxed afterwards. In Europe after a blue pill various dimensions are checked before and after firing of a blue pill, so it's much more involved and stringent.
 

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Kevin is right again.

In the US proof is "voluntary" but if you want to be member of SAAMI (highly recommended) you have to play by their rules.

In Europe proof is mandatory and CIP (congress internationale de proof) sp? and their testing methods are a bit different.

US proof loads can have a different powder, more of it, or different bullets. One of the coolest is the .30 Carbine which used a 150 gr. M2 ball bullet.
 

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My Recollection...

My understanding is that the cartouche is an inspection mark, not a proof mark.
...is that the cartouche was considered an acceptance mark, at least for firearms not built in government arsenals, such as the M1 Carbine. However, it seems logical that it would have been a carry-over from similar inspection at the government arsenals, where it may well have been considered an inspection mark.
 

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...is that the cartouche was considered an acceptance mark, at least for firearms not built in government arsenals, such as the M1 Carbine. However, it seems logical that it would have been a carry-over from similar inspection at the government arsenals, where it may well have been considered an inspection mark.
I would say that's a more accurate description, acceptance.
 
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