most likelywont loose any velocity,

heres a quote from swat magazine on a test that tactical operations did on barrel lengths and velocitys with a 300 win mag

To find the answers, Tac Ops took a 26-inch barreled .300 Win. Mag. and chopped the barrel down in one-inch increments as they previously did with the .308 Winchester. Ten rounds of Federal Match 190-grain BTHP Gold Medal were fired from each increment. No velocity was lost from 26 inches to 22 inches. Velocity loss started to occur only after they went below 22 inches

to read the full artical go to
http://www.tacticaloperations.com/

then to articals and the SWAT dec-2000