This happened to me this weekend. Every once and a while (3 times out of about 100 rounds fired) the spent case would fail to eject and as the bolt came back forward it would pick up the fresh round, attempt to feed it but push it up into the spent case locking up the weapon.
When I got home and was cleaning up I compared the action spring to a new one and the one that came with the lower (BCM) was considerably shorter than the new one (also BCM) by about 2 inches. Now the spent rounds were ejecting pretty hard. Do you guys think the action spring might have been the problem?
Here’s my set up:
Upper: BCM 16" carbine gas system upper
BCG: BCM
Lower: BCM Carbine
It’s a carbine action spring from BCM I even went back and verified the order and matched it up to my son’s M&P 15 spring and sure enough the new spring is the same length as his.
If your spring is that short, try that other Action Spring you compared it to and see if it fixes problem.
Beyond that: What ammo used? What Mags? How many rounds through this upper? What buffer?
That’s my plan this weekend. As for the other things;
Ammo: PMC 62 gr Xtac
Mags: MAGPUL 30 rnd PMags (this functioned fine in my sons weapon) #Rnds: MAYBE 175 now.
Buffer: H Buffer
Was this Action Spring one you purchased from BCM? Is it new? If it is that short, you might send it back and exchange it.
Also, (if it not the Action Spring) and you only have 175 rounds through this upper, you might be just experiencing some of the teething problems that a new upper sees when “breaking it in.”
The one that was giving me the issue is the one that came installed in the lower. The one I intend to try this weekend (after measuring both tonight) is new from BCM. I thought about the “Break in” thing as well but was interested in any input other members might have.
So I get home last night and pull out the old action spring along with the new one and sure enough the one supplied with the lower measured 10 1/8" and the new one I installed 11 9/16". I verified this with two seperate rulers and a tape measure. I have to admit this kind of surprises me given BCM’s reputation that the spring supplied with a new lower would be so short (even though it is still within spec). Not only that but working the action manually with the new spring is considerably smoother now. I gues I’ll find out this weekend if it has any effect on this issue.