Question for everyone who has built their own AR or replaced parts. I just purchased the daily deal from PSA which is an LPK and a couple mags for $59.99. My question is, can I drop an LPK from PSA into a RRA AR15 without problems? Reason I ask is because I bought some Knight Armament guard rails for my RRA carbine and they don't fit. I wanted to have some extra parts for the lower in case they needed to ever be replaced. Or, if I decide to buy a stripped lower and put one together myself. Just curious if anyone knows if the LPK's are all interchangeable regardless of the AR manufacturer. I'm also looking at buying another AR from Armalite. I'm assuming they are all the same but would like clarification since I've never actually replaced parts on an AR before. Thanks.