Not a good idea to try and use any of the Combloc ammo in our 7.62x51 NATO chambers. 

 

U.S. round is 7.62x51: .308 bullet and 2.007 case.

Combloc 7.62x39: .311 bullet and 1.535 case.

Combloc 7.62x54R: .311 bullet and 2.126 case.

 

The 7.62x39 is short enough, but the bullet is a .311 vs. .308 for the 7.62 NATO.  The 7.62x54R is a rimmed round and won't fit at all and the same bullet diameter holds true.  The NATO round would fit a 7.62x54R, but it won't chamber (case tapers are different) and it wouldn't extract because the NATO round is rimless vs. rimmed.

Negative on both ways.  Chicom and NATO rounds are expressed in metric terms: (1) bullet diameter in millimeters, (2) case length in millimeters, and (3) any special characteristic [rimless no suffix, rimmed - R suffix, rebated rimless - RB suffix, semi-rimmed - SR suffix, and belted - B suffix]. 

 

The Chicom rifle rounds are 7.62x54R and 7.62x39.  The U.S. round is the 7.62x51 NATO. 

 

Bullets on the Chicom rounds are .310 to .311; our bullets are .308. 

 

The problem is that the cases will not chamber because they are completely different in shape and length. 

 

The 7.62x54R is a rimmed case dating to 1891 in design and the 7.62x39 is a rimless case dating to 1943.  The 7.62x51 NATO is a rimless case dating from 1956.

 

Case lengths:

7.62x54R -- 2.126 inches

7.62x39 -- 1.535 inches

7.62x51 NATO -- 2.008 inches