Bolt fails to lock back on empty mag

AR/M4 AR Technical Discussion
1

About 3/4’s of the time when I fire the final round my bolt will lock back on the empty mag. The other 25% of the time the bolt returns into battery obviously with no round chambered.

I ordered a replacement bolt catch, spring, and plunger/detent. Replaced these. After having replaced these LPK components the stated problem still occurs. ???

I have 15 PMAGs (30rounders). 10 purchased at one time all have same date stamp of manu. The other 5 purchased at another time all have same date stamp of manu. Occurs randomly with any one of the 15.

What to do?

originally the CTR setup I bought came with an H2. purchased an H to see if it would have any bearing. MidLength. Mostly PMC bronze, Federal LC, and Silver Bear. CMMG upper
Happens with both buffers and all flavors of ammo. thx

2

Quite possibly a short stroking issue?

3

What weight buffer? What gas system? What ammo? What make of rifle?

4

C buffer is gonna fix this… midlengths dont need H buffs.

5

Try same mag and same ammo in another known good rifle. If it does lock back every time on the last round at least 10 times in the known good rifle then you likely have a gas leak somewhere.