So I bought a 458, with a 20 inch barrel today....

[Letzhunt]

I wacked 3 1/2" off of my 458 WM+ (3.6" mag box) Winchester Safari. I was getting 2410 fps when it was 24". I am hoping to get around 2300-2350 now out of it at 20 1/2". I am shooting 450 North Fork SS and flat and cup nose solids. I use both TAC powder and A2230. Even if I only get 2275 FPS it will certainly do the job needed at the ranges I will be shooting it. My only fear are the Swarovski scope and walnut stock handling the recoil long term. You should be able to find as great load for that rifle. I think you likely limited on COAL with the magazines it uses. But NEW factory ammo is good. Try Buffalo bore with there 400 Barnes or the 450 Barnes.

 

[Andrew62]

I had the barrels of both my .375H&H and 416 Rigby cut down to 20". At buffalo/elephant ranges, which is what I use these guns for, I cannot see a meaningful reduction in velocity to matter.

 

[hoytcanon]

I have half a dozen big bores with barrels from 18" - 20"... no issues, plenty of power but much handier than a 24" rifle.

 

[Hyde Hunter]

Buffalo Bore has 400 TSX loads at 2650 fps