I just got done test firing my first AR build and the bolt won't stay back after the last round is fired. I checked the mags and they're fine. I don't think it could be the bolt or buffer being too heavy because it cycles just fine. If I pull the bolt back and hold the catch out then it will hold but only until I take my hand away, then it slams home again. I'm thinking the spring on the bolt catch is too strong? Any thoughts are greatly appreciated.