Neck tension

[ported45]

I tried the Lee factory crimp die with various levels of crimp and it didn’t make a difference

I had the exact issue while trying to get BCA uppers to work out for me. I set the LFCD from light to tight enough to create a crimp ring in the bullet when crimping and the bullet still would slide forward in the cartridge when chambered. When I finally gave up on getting a "good enough" upper from BCA, I ordered a Hardened Arms upper as I had 5.56 and 300 Blackout uppers from them that shot bug holes. I had absolutely no bullet slip with the Hardened Arms upper even without using the LFCD at all. I don't know what it was about the BCA uppers, but they just sucked all the way around! Changing uppers solved my bullet slip problem.