That's not true. It's pure myth. Though a long barrel's returns are diminishing, you will never see a gun that actually retards bullet speed and accuracy through barrel length. Yes, a bullet out of a smoothbore gun of the same caliber, load, and barrel length will fly faster. It'll also fly very far away from where you want it to.
Anyway, if the XM8's barrel length is less than that of the SG551, it should have lower penetration and less damage. Maybe not appreciably lower, but I haven't looked at the numbers. As for recoil, I can't test it right now (uninstalled UT), but if the XM8 does have less recoil, that does need to be fixed at some point. Lower muzzle velocity does not decrease recoil. They are firing the same ammunition. For the purposes of the abstraction of the game, the only appreciable factor in recoil, at that point, is weapon weight.
Anyway, if the XM8's barrel length is less than that of the SG551, it should have lower penetration and less damage. Maybe not appreciably lower, but I haven't looked at the numbers. As for recoil, I can't test it right now (uninstalled UT), but if the XM8 does have less recoil, that does need to be fixed at some point. Lower muzzle velocity does not decrease recoil. They are firing the same ammunition. For the purposes of the abstraction of the game, the only appreciable factor in recoil, at that point, is weapon weight.