The differences for big game between 300 Win Mag and 300 Wthby Mag, are $1 per shell, compared to $2 per shell. That's about it. And maybe 100-200 fps at best, if loaded with the heaviest bullets, otherwise they are identical.
the reloading specs on some 300 mag family cartidgee are exactly identical, even though the case volume is a lot different. For instance 308 norma mag vs. 300 wthby mag, exactly the same powder charge for 150 grain bullets, for some powders.
we're talking the same girl in a different dress
It's really a question that borders on nonsensical, because you can fire 300 Win Mag ammo, in a 300 Weatherby. If you buy a 300 Weatherby, you basically get a 300 Win Mag and 300 H&H included, free of charge- because it will fire all 3 cartridges just fine.
no different than shooting 38 special out of a 357 magnum handgun, with the shell headspacing on the rim. In the 300 belted mag family, the shell headspaces on the belt, the case just fireforms to the chamber. If the caliber is the same, and the case is long enough to put the bullet tip into the throat lead of the barrel, it will fire out of the gun.
if one already owns a 300 Wthby Mag, it makes absolutely no sense to buy a 300 H&H or 300 Win Mag, or even a 308 Norma mag. If you want to try out a 300 H&H or 300 Win Mag, you can just buy a box of shells and shoot them through your 300 Weatherby. They are all basically the same in terms of performance. If one wants to try first time the 300 mag family and doesn't want the wallop, blast, recoil of the 300 RUM or 30-378, the 300 Weatherby is the only choice, and best choice, for this reason.
I would not magna port any rifle barrel, the blast effect on your muzzle-side ear would be dramatic, and your ear would ring for 3 days from firing one shot, or worse. It would require earmuffs just to hunt with. The dirty little secret about porting any barrel is, recoil is reduced, but muzzle blast is greatly INCREASED. But again, it gives the firearms companies one more thing to sell you.
You're actually recommending the practice of firing a .300 win mag in a .300 Weatherby?
