Weatherby chambers have more "free-bore". Free-bore allows for more powder burned at similar pressures. Don't make the mistake of chasing high velocity. The 7mm RM is a superior cartridge to the 7mm Wby. More powder burned equals more barrel erosion per shot. The 7RM uses 5-10 gr less maximum powder charge than the 7WBY.
That used to be true when Roy Weatherby first started to promote his cartridge/rifle combos but about 20 years ago and likely more that changed. There was a good reason for the change. Custom rifle makers started cutting chambers with less or even in the case of customer requests for specific hand loads almost no free bore at all. This was done because accuracy enthusiasts weren't comfortable with relatively mediocre results of factory Weatherby chambers. As always, there were exceptions to the rule, but not many.
Recently I sold a lovely vintage Weatherby in 300 Wby Mag. It was one of those wonderful pre MkV Mausers. This one was a commercial long action with a square bridge, Triple A walnut stock and a very nice Weatherby scope that was mounted later. Beautiful rifle. The fellow I bought it from couldn't get it to shoot well and it sat in his safe until he passed away. He just couldn't bear to part with it even though he never took it on a hunt. I held onto it for years. It was a beautiful rifle and impressed me every time I picked it up.
When I bought the rifle from the estate, it came with loaded cartridges, boxed bullets, powder, cases and dies. A few months before selling it I thought I should shoot it and actually take it on the hunt it had been built for. The older cartridges shot into 6 inches at 200 yards at best. I pulled all of the bullets and dumped the powder/changed out the primers and loaded them up with more modern components.
The bullets from the sixties just weren't built to the standards we enjoy in the modern bullets we have today. No matter which 30 cal rifle I shot them out of, 4 inches at a hundred yards was about the best I could get out of them.
The load I used:
200grain spire point
85grains of Reloader 25
CCI 250 magnum primers.
The bullets were seated out as far as the mag would allow and there was still at least a half cm of freebore. Freebore isn't always detrimental but when it is sloppy and over diameter it is a killer as far as accuracy is concerned.
Even with modern components of proven quality the rifle just wouldn't shoot. It never did get to go hunting while I owned it.
The rifle went to a Weatherby collector that will likely never shoot it.
Sorry for the hijack. I just felt you needed a bit of background on Weatherby cartridges. The reason Roy Weatherby used so much free bore in his chambers was to allow some of the gasses to bleed off and reduce pressures. Today this isn't necessary because of powders being available that are more suitable for the cartridges.
Now, as mentioned before the pressures between the 7mm Weatherby and 7mm Rem are very close. Within a couple of thousand CUP. Certainly not enough to give you any realistic increase in velocity and certainly not enough to make any difference on big game. The high pressures generated could be detrimental to accuracy and certainly be harder on your rifle and body.
