We have put out five 7mm Practicals (7mm-300) over the last year and generally, I work on client 7mm Rem mags on a month to month basis. The average velocity gain for the 7-300 over the 7mmRM is 125fps.
The 7mm Weatherby produces pretty much the same velocities as the 7mm-300 via the use of free bore. The Weatherby case is roughly the same dimensions as the Rem Mag case but has a quarter inch bullet jump to allow extra gas expansion in order to create higher velocities. If the 7mm rem Mag is given a quarter inch freebore/bullet jump, the velocities are the same. That said, there is one major difference, the Weatherby cartridge is designed with a very long case neck to aid bullet to bore alignment. This goes on from what I was saying in a previous post about the subtleties of cartridge design. Don Allen did the same with his Dakota cartridges, these also have a quarter inch bullet jump but in order to counter yaw during the free flight from the case to the rifling, the case necks are very long.
The 7mm RUM by contrast has a short neck and .400 thou jump, nearly half an inch. This works OK with 180 grain bullets but some rifles simply won't shoot 160-168 grain bullets as the projectiles enter the rifling off center after completing the free jump. If you drop a 162gr A-Max into the chamber of a 7mm RUM rifle, then put a case behind that and close the bolt, the bullet will rattle both forwards and backwards, as well as side to side. There is quite a bit of room in the RUM throat- all designed for safe gas expansion towards the goal of highest possible velocities.
If the throat of the Weatherby is shortened, it will duplicate the 7mm Rem Mag. If you shorten the throat of the RUM, it can be dangerous with some loads due to the lack of room for gas expansion. I have documented experimentation with this on our site. Another set of documented results regarding overbore pressure problems can be found in Australian Sporting Shooter. Editor Nick Harvey built a .25 WSM with minimal freebore, the rifle reached dangerous pressures with what should of being mild loads, had one sweet spot, then back into high pressures when loads were increased. These types of results are not one off's, they can be repeated. Safety first.
When I designed our Practical reamer, I wanted a minimum freebore cartridge to remove one set of variables. Advised bullet jump is 40 thou. The design allows users the choice of a variety of bullet weights and styles. All of the five current rifles produce .3MOA or better. It is certainly not the only way to go and freebore isn't what I would describe as being of major detriment. There are three very common cartridges which I have to work with regularly for clients which utilize long freebore in their deign, the .308 Win, the 6.5x55 (Carl Gustaf rifles) and the 7x57. All are of course capable of excellent accuracy. However, with magnum powered cartridges, at the very least, an understanding of how freebore might effect results can be useful in determining how best to design a cartridge (such as including a long case neck), how to utilize the cartridge, choice of twist rate and how to troubleshoot problems encountered.