7.62x51 is the NATO round. The commerical .308 Winchester round is basicly the same dimentions as the 7.62x51 but has higher overall pressures. (Off the top of my head I think the 7.62 is around 50,000 vs. the .308's 68,000) This has been argued back and forth, especially when used in some of the Spanish military surplus bolt rifles. I've used both in my guns, including my Spanish FR-8 and never had any trouble.(I have heard that there is a Spanish 7.62 round used for some of these rifles loaded to lower pressures, but I have never seen it anywhere or really ever discussed, so I'm not sure it even exists) When you reload 7.62x51 military brass, you do need to load it down one full grain from the same load using commerical brass due to pressure issues according to most manuals. I think if your gun is in good working condition, you wont have any trouble.