The Argies used 7.65 x 53 chambered mauser rifles. I never understood the 7.65 x 54 designation for their ammo. Here is a possible explanation stolen from another forum:
http://parallaxscurioandrelicfirearmsforums.yuku.com/forum/viewtopic/id/9659
Quote: "I've often wondered why the "modern" Argentine surplus ammo was headstamped 7.65x54 when the original cartridge designation is correct at 7.65x53. The nominal case length is supposed to be 53.4mm I believe, which rounds down to 53 conventional. But supposedly the Argentinians produced these last lots of milsurp ammo to help in the sales of their surplus rifles long after they had standardized on (Nato?) calibers, and left the 7.65 behind.
Somewhere, I read that their case making tooling was out of tolerance from wear, but they didn't want to retool for non-critical surplus sales, so they ran the production with what they had and it came out a bit longer than nominal specification. Perhaps they also didn't want to get it confused with their supplies of true Milspec ammo, and as long as it was a bit oversized they went ahead and rounded up to 54, which would be correct for 53.5mm and above.
Realize that a difference of 0.1mm is a bit less than 0.004" (four thousands of an inch) or about the thickness of a sheet of paper, and you can see it wouldn't take a significant case lengthening to legitimize the new nominclature. Also realize that even SAAMI specifications on cartridge and chamber dimensions allow some tolerance in every dimension, so by exceeding SAAMI specs a mite in the case dimension, the ammo was still likely to fit most chambers, but not the very shortest ones at the bottom of the tolerance. And of course you should also realize that Milspec ammo was never produced to modern commercial SAAMI specs anyway, as they mostly predated SAAMI, and were internalized to work from their own military specifications and tolerances."
