It may also surprise you to know that it is possible to shoot the same load in two rifles with different barrel lengths and produce a higher velocity with the shorter barrel.This is because the chamber and barrel internal dimensions are also a factor in producing velocity.A tighter chamber will produce higher pressure which will usually produce more velocity.If the shorter barreled rifle has a tight chamber and the longer barrel has a loose chamber,the pressure difference may more than offset the difference in barrel length.By the way,the barrelthickness is not a significant factor where velocity is concerned.