Have you slugged the bore? It might be undersize, causing pressure issues. Once I loaded ammo for a fellow with a beautiful Griffin & Howe custom 270 on a Mauser action. It had all the bells and whistles. However, my standard 270 load (55 grains of IMR 4350 under a 130 grain bullet) was too hot at about 3,150 fps. I backed down, and ended up several grains under what I started with. I didn't slug the bore, but have little doubt that was the problem. An undersize chamber was not the problem.
