I purchased a BCM Barrel and it was a designated marksman's Barrel made out of 410 stainless. I can only get one brand of ammo to feed. I had to ream the chamber out a bit to open it up and then I had no more problems out of it . I'm not saying that as your issue but it could be something to that effect.
It is certainly possible. I'm going to load 10 rounds in this 1x fired Lapua and go test tomorrow at my local range. If that ammo works fine I think I will lean towards the small base die versus modifying the chamber.
