widowmaker972

I have a a PA10 upper with a matched PA10 lower SR25, been using ASC SR25, 20 round mags. When I load the mag with 7.62x51, chamber a round, and pull the trigger, I get nada and the bolt locks up, I have to take a hammer and pound on the charging handle back to get the round out of battery. This does not happen with .308 Win. What's the deal?, I thought 7.62x51 and .308 win were the exact same round with the load being the only difference. Help !