velocity loss with barrel length reduction

[JohnC]

just got back from a muzzleloader hunt in Dryden. found that the barrel length on my Remington in-line ML (24") was a bit awkward & am considering getting it chopped to 20". How much muzzle velocity loss can I expect & can I compensate with a higher load - now using two 50-grain pellets of 777 with a 260 grain saboted projectile.

comments, replies gratefully accepted.