Tom Bigbore has made a good point here davem3.

Ruger's barrels have .458" to .4585" groove diameters in both the .45-70 and the .458 Win Mag of the ones I've measured. BUT, the .45-70 have a twist rate of 1 turn in 20" vs. the .458 being 1 turn in 14"

Tom's metric chart shows the groove diameter at .457" (11.61mm) and the twist at 1 turn in 15" (381mm) for the .450 NE.

While the 1 in 20" twist may keep 500 grains point on on paper, both the .458 Win Mag and the .450 NE have the quicker twist for a very important reason. To keep long solids point on penetrating dangerous game.

BigRx