As I understand it, they are the same but perhaps somehow different also.
When I fire surplus ammo in my rifle, the bolt is much harder to open.

Im guessing it has to do with thickness of the material in the case neck. Or maybe that army ammo is made of softer brass that expands more, or maybe dont flex back so much after firing, or something.
I dont use surplus ammo anymorebut have never heard of any other problem, exept poor precision.
I hear some people even use it in 30.06 rifles, but that cant possible be sound.