RRA 20" A2 about 1000-1500 rounds through the tube. 30 rounders fully loaded XM855PD. All mags USGI type various mfgrs some new, some used. When manually pulling back and letting it slam home the bolt starts to strip round and stops with round on ramp? It doesn't happen with all mags but mags I thought were good jammed the same way??  While checking for carrier binding I found that at a 45 deg angle letting the bolt and carrier fall on it own weight it would not lock fully into battery. The bolt is fairly tight in the carrier, but clean and no damage to rings? Buffer inspected and it is clean not damaged and well lubed, slides fine. When pushing down bottom of bolt catch and pulling back charging handle no binding.

Sorry so long, I'm trying to cover many possabilities. Any help or advice would be appreciated.

RRA warranty is my next option. hug.gif

Maybe a bent firing pin or twisted CAM pin?

Ive encountered similar trouble from certain mags that are loaded to maximum capacity, but it didn't happen on a regular basis... Ive since started a new practice of only loading my 30 round mags with 28 rounds and never experienced the trouble again...

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with bolt out of carrier, resistance is increased when carrier key meets gas tube.  
No damage to firing pin or retaining pin. light wear mark in middle on finish of retaining pin but on damage.

no binding until getting to the gas tube. upon closer inspection the carrier key has bright metal on the lower lip idicating to me that it is hitting the gas tube on the lower side. I adjusted gas tube a little and I think it is somewhat better, but I can still feel when the carrier key meets the tube. I didn't want to get too liberal with the adjusting so..?  The carrier will now fall from 40 deg. and close on it own weight. After testing I pulled out a mag that had this problem and it still happened. I pulled the mag down a little and the round chambered.. I will have to try more mags tomorrow. Tweak Thanks for all your help and insight.

Resolved:
Mags--
"KNOWN GOOD" not so much..  brand spanking new in wrap-- meaning not cleaned, lubed.
mouth of rounds were hanging up on front side of mags. I was focused more on the carrier and ramps. I filed sharp edge down, cleaned with warm soapy water, CLP'd, reassembled, tested 100% on three of the mags that were failing. I never saw anyone state that mags may need a little filing on the front edge, I guess that firing would take care of it eventually..
Noobies do learn Thank you to all who responded.