I built a 16" AR15. Most Parts are BCM with exception of my AR57 lower and my generic LPK (both bought before I did my researched and learned that all AR parts are not the same). Everything was running smoothly after about 500 rounds. However, this weekend I went to sight in a new scope. After 10 rounds I heard an odd noise then my weapon discharged as my bolt drove a round into the chamber. I dropped the mag and upon inspection my hammer pin had worked its way out of the safety side of the Lower receiver. Nothing broke and I was able to break the weapon open and reinstall the pin.
Question: Faulty lower, faulty LPK, or installation error?
Rookie Mistake: Posted then searched. I’ve found several possibilities but any info is welcomed.
More than likely you have a shit lower parts kit. Hammer pins are retained by the spring inside the hammer called a “J” spring. If it is missing or crap it will allow the pin to walk loose. I have seen hammers that were missing the “J” spring. You should start there. The other possibilities are incorrect installation or potentially out of spec lower.
It was ar57 lowers made by Aero Precision that were way out of spec (in numerous ways depending on lower). Guess AIM surplus had a crap ton of them that sold having the issue. A google search of this will bring up numerous threads on a slew of forums. Not just a little out of spec, we are talking way fugged up in numerous ways. AR57 was also having spec issues with their uppers as well and I believe still are. Seems since AR57 decided to come out with their gen 2, everything started getting pretty bad.
I know one of my Aero Precision lowers is pretty out of spec on the diameter of both pairs of pin holes but placement seemed fine at least :suicide:
I’ve also seen a pics of discount lpk’s that don’t even have the cutout in the middle for the j pin. Guessing they forgot the hammer pin should always have one while packaging them up.
I recently took the old internals and replaced them with an ALG Combat trigger. I have not had a chance to play with it yet but hopefully this solves the problem.
while that might prevent this problem, you might be setting yourself for another , more serious problem, such as a dead carbine should a primer pop out and jam the FCG. that is, unless you carry 2 correctly sized allen wrenches.
