Bear Creek Arsenal Complete Uppers

[ported45]

I'm sorry for not understanding exactly...so when the round chambers the bullet is sliding forward into the rifling due to the sudden stop of the cartridge?

Yup. Using Hornady dies with 0.262" elliptical expander that leaves the case mouth at 0.258" before inserting bullet. I brush out the necks before sizing, just like all the other brass I ever use. I ended up putting a pretty good Lee FCD crimp on it to keep the bullets from moving forward when chambered.

I also load 5.56 and .300 Blackout for the AR and never had any problems like this before using the same reloading technique and Anderson complete lower.

I am going to work on making sure that the case mouth has been cleaned and degreased to eliminate any friction loss due to leftover residue or case lube that could have (however unlikely) got in the mouth of the cartridge.

The guys here are saying that with 0.258" ID neck should be plenty to hold the bulllet without any crimp at all. It was also suggested to check the Anderson carbine spring for appropriate # of coils. Guess I will have to Google what would be appropriate.