The controllable nature of mp5 is not because of roller delay system. It is because of it is not overgas. Most people think m14 full auto is uncontrollable but if you install a good muzzle brake and tune the gas you can shoot it single hand and still control it. 7.62 x 39 does not kick as bad as most people think. Ak is overgas if you shoot cmmg mk47 chambered in 7.62 x 39 where the gas system is properly tuned it does not kick as hard as ak. Also one big reason that police is switching to short barrel rifle is smg does not have the stopping power to stop drug addicts.
I don't think the main reason why the police's G3 can't hit accurately in 30 yard is because of training. I think it is mostly due to gun not properly maintained. It is like Beretta m9a1 pistol is most US military can't hit anything because the barrel is shot out.
