I am trying to diagnose a problem with a new upper receiver that doesn’t keep the bolt open after the last round has been fired. The problem always occur with the PMAG M3 and not with the Lancer L5 AWM. Both magazines are 30 rounders.
The upper consists of the following:
Vltor MURA
Young Manufacturing M16 chrome BCG
Lilja 16” carbine gas length
Geissele Super gas block (gunsmith pinned)
Ammo: Lake City XM855
Symptoms:
Both PMAG and Lancer magazines with 5 rounds loaded cannot be easily inserted into a close bolt
Shells are being ejected at the 3’o clock position
No fail to load or fail to fire incidents
Observed problems:
Both PMAG and Lancer magazines with 5 rounds loaded cannot be easily inserted into a close bolt
PMAG never keeps the bolt open after the last round has been fired
Lancer the bolt remain open after the last round has been fired
So what was is the lower's configuration and how does that work? The answer is that it functions superbly.
Same ammo and magazines.
Zero symptoms with shell ejection at 4:30 o’clock. Magazines can be inserted into a close bolt – no problems.
The lower consists of the following:
Mega Arms billet lower
Spikes Buffer ST-T2 (4.1oz)
Sprinco blue buffer spring
The upper consists of the following:
Mega Arms billet upper
WMD nickel boron M16 BCG
Rainer Arms 16” mid-gas length
Geissele Super gas block (gunsmith pinned)
Any suggestions on the next step?


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