What cartridge is this?

[Schrodinger's Cat]

(7.62x54R on the left, mystery cartridge on the right.) Was at USI this morning with my Mosin and I found this in my box of yugo 54R. Damn, I almost loaded it up! I'm not sure if the bolt would have closed...but nonetheless. The casing is thinner and longer but the bullet is fatter than my 7.62 shown on the left.

Is this to be expected from repacked surplus or would it courteous to inform the place where I got it?

[AJAX22]

Looks like an 8mm Mauser round to me

[f4tweet]

http://cartridgecollectors.org/headstampcodes.htm

[Mssr. Eleganté]

Looks like an 8mm Mauser round to me

Yep, looks like 1950's Yugo 8mm Mauser. The 12 on the headstamp means it was made at Prvi Partizan in what is now Serbia. That's the same factory that made the 54R round in your picture.

[Whiterabbit]

you can put the calipers on it. mauser is 57mm, so if that case is 57mm and the bullet 8mm, you know it's 8x57.

[jwest]

http://en.wikipedia.org/wiki/Schr%C3%B6dinger's_cat

[Schrodinger's Cat]

[Whiterabbit]

An animal that lived a full and healthy life, dying of natural causes, dammit.

[kurac]

+1 for an 8x57 mauser round, also called a 7.9mm x 57mm

[mif_slim]

8mm vote here too!

[CSACANNONEER]

8mm gets my vote too.