Weatherby rifles and cartridges are famous for their sizzling hot muzzle velocities. They do this with a longer throat or free bore than a standard rifle to allow the bullet to get a running start at the rifling and thus lower the chamber pressure. You also wont see any standard Weatherby rifles winning any bench rest competition with these long throated rifles because they are not as accurate.

I have a AR HBAR with a 1 in 9 twist and a bull barrel .223 bolt action with a 1 in 9 twist and they both like 55 grain bullets and are very accurate.

In my opinion the military didn't do anyone a favor by going to the M855 round with the M193 round being more accurate and having better stopping power. On the flip side of this the troops in Viet Nam complained that the M16 lacked penetration when firing through obstacles, barriers, etc compaired to the M14 or M60 which would go through them and hit the VC or NVA.