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Test fired my Aeroprecision 8" M4E1 upper yesterday using 180 gr loads from Thunderbird Cart Co. Loaded single rounds in 30 rd Magpul M3 windowed mags for initial sight-in, and to insure the BCG was being locked back. Gas port is .109", using Tactical Spring Co "Hot White" spring and an "H" buffer. With open sights and a front bag, was able to get ~ 1" 3 shot groups at 50 yds with variable cross-winds using MBUS Pro sights. Loaded single rds the entire process (20 rds) and all locked back and grouped well. Happy so far.

Not wanting to continue, I loaded 5 rds to test feeding. Rapid fire from the hip, the bolt did NOT close on the 4th rd by almost 1". Ejected that rd, and I could see that the 5th rd had started to move out of the mag, and was stopped by the feed ramp. The 5th rd moving forward jammed the bolt, and was the cause for the 4th rd not chambering.

I measured the 300 Blk rds at 2.050" OAL, compared to 55 gr .223 at 2.190" OAL. The 300 BLK is .140" shorter, and does not always seat to the rear of the mag. I would assume they move forward under recoil regardless of how you load them.

Maybe I just need to cycle it more, or is there some glaring issue here ?? I need this to be a dependable PDW.

Thanks !!
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