Practicality factor: HK MP5/10
The only reason I would really want an automatic weapon would be a sub-machinegun for engaging multiple close range targets in a home defense situation, at longer ranges I'd probably stick to semi-auto with a larger caliber .308.
Historical factor: German MG42
This gun was the equivalent of the Henry lever action rifle in the Civil War. It was the first true assault rifle, and the standard military arms of the next century followed its basic format: full-automatic capability, detachable high capacity magazine, high powered rifle ammunition. Though ammunition would be a PITA to procure, I would not shoot the gun that often. Though should a bad guy come knocking, I think I'd jump at the chance to let it fight for the good guys for once in its life.