Shinjidude said: bayonet (n): a daggerlike steel weapon that is attached to or at the muzzle of a gun and used for stabbing or slashing in hand-to-hand combat.
I'd assume its something to do with the propellant gases and how the bayo prohibits them from expanding downwards when the bullet exits the muzzle. But really, that's just a guess.
I'd think it more to do with the barrel flexing slightly downward due to the weight of the bayonet. A human wouldn't notice the flex with their naked eye, but the flex would definitely affect the rifle's long-distance accuracy.