I think the MP44 / StG44 actually brings a little balance to the game, to offset the Garand (vs the bolt action K98). The mod that allows you to run a little faster with the 44 is CKR, the reason being is that it is a different arm than the BAR. When the movement speeds were adjusted, it wasn't only for the weight of the gun, but also it's "speed".
By that I mean the relative amount of time it takes to put the gun on target, manuever in close quarters, etc. This is not a concept from computer games, but from the real world. There is usually a trade off that occurs with speed vs accuracy. The less speed, the more accurate (as in a rifle). The more speed, the less accurate (as in a pistol). In this sense, an assault rifle is "faster" than a battle rifle. Since there's no way to adjust for this with the game's settings, it comes out in movement speed.
------------------
"If your philosophy does not produce corn, I don't want to hear about it." - Red Bear