I am building a 458 win mag bolt gun. I have a 24" barrel and looking to shorten it. I am wondering if anyone knows or has done a velocity test on various different lengths. I'm thinking this would be a lot handier with a 21", maybe even a 20" barrel. What would be the performance loss?

I have found that for every inch of barrel length you get an increase or decrease of around 35 fps in the 458 win but powder choice will have some influence on this.

If you get 2,150 fps with a 500 grain bullet from a 24" barrel you can expect 2,080 fps from a 22" barrel, 2,045 fps from a 21" and 2,010 from a 20" barrel. If your hunting buff and using softs probably no problem as it would be for elephant heart/lung shots. If using solids on elephant heads, then 2,050 fps is very marginal but 2,150 fps is excellent for deep penetration.

465H&H

2150 worked for my 500NE last month in Botswana, no bullets recovered.