No. You need to bore out at least 55 thousands otherwise you will get "reverse rifling". The drill will chatter on the existing rifling and produce ghost riling in reverse.

How do I know, I asked the man who re-bored my .303 to .358. Also you will have to get it re-regulated.

I know you question is hypothetical, but I also have a .375 H&H belted DR and love it the way it is. In fact, instead of of amping it up, I back it off to flanged velocities to save the action.

Which is a great seqway(sp) for belted DR's and modern bullets. The predominate issue with belted DRs is the high pressure stretching that action loose. However with modern bonded and mono bullets, one can reduce velocity and still retain good penetration. A slight(~300-400 fps) reduction in velocity yields a large reduction in pressure(~3-4 tons).

This makes belted DR practical. So a new DR in 400 H&H would be super cool, as long as you back off the charge just a wee bit.

B.