Sounds to me like perhaps your recoil springs are weak. I'd replace the recoil springs and guide rod. Also make sure that your gas system is properly regulated for whatever type of ammo you're shooting. It is possible that one of the previous owners shot hotter ammo than what the rifle was set up for. The gun will function with heavily loaded ammo if the gas system is set up for light ammo, but you run a higher risk of peening the reciever (not to mention it will kick like a mule). When adjusting the gas system, you want to allow just enough gas to get to the piston to reliably cylce the action but no more.
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