AA Lapua 123 Scenar ammo won't feed but Hornandy will?

[jurassic]

Thats going to be the next thing I do. Will knocking off all the sharp edges on the front of the barrel extension cause any problems?

The sharp-nose HPBT's often get hung-up on badly-fit or blended uppers-to-M4 extensions, which is why I prefer to blend and polish them, while taking the edges off the barrel extension teeth that contact the projectile and case.