I just assembled my first lower and took it out to function test.  Everything works fine except the bolt hold open.  It worked on only 1 of 6 "known good" magazines.  When I seperate the lower to visually check function with a magazine, iot appears to work.  Any ideas on what could be causing this problem?  By the way, the upper used is an 11.5" Bushmaster, the lower is a Bushmaster and the lower parts kit was from CMT/Stag.

What ammo were you shooting?


Check your carrier keys.

make sure you have a regular carbine buffer, my last build did that because i had a heavy buffer, make sure the buffer is a regular carbine buffer!

Thanks for the quick replies guys.

The buffer and spring used were standard carbine parts.

The key is good and tight, no wiggling at all.

The ammunition used was British RG surplus.  I have used the ammo from this lot in a 16" and a 6.5"(pistol) without any function problems.

Problem solved.  After reviewing gun maintenance records for my other carbines, it appears that the spring I thought was a standard carbine spring is in fact a Wolff +10% spring.  Went back to the range with a standard spring canibalized from another carbine.  Lo and behold, it works fine with a standard buffer or a 9mm buffer.  Lessons of the day: Keep good records, mark what is what and 11.5in AR's don't like extra power recoil springs.