Rheinmetall’s plans for the high-performance .50 were to match the performance of the 14.5mm KPV while the gun could still use standard .50 BMG ammo. Target ballistics were stated as 40-42 g bullets fired at 1,100 m/s or 50 g at 1,000 m/s. This involved a much higher chamber pressure (unspecified) and the use of steel cases. The cartridge case was supposed to be modified so that it could not be fired in a standard .50 gun (which would probably blow up). Exactly how this was to be achieved was never specified, but obvious alternatives are a case with slightly less taper or a sharper shoulder angle, or maybe a semi-rim.
The idea was to offer a longer barrel chambered for the high-pressure ammo (which could also fire .50 BMG) or a standard .50 barrel for anyone who preferred that. I say “was” because the whole project - and especially the high-pressure aspect - seems to have gone quiet.
The choice of external power for the 50 RMG was inevitable, I think, because a gas or recoil action would have real problems functioning reliably with both high-pressure and standard ammo; the internal ballistics would be radically different. Given that the main intended use was in AFVs I don’t see the dependence on electrical power as a problem. The most popular AFV cannon in the west - the Bushmaster Chain Guns from Orbital ATK in 25mm, 30mm and 35mm calibres - all are driven by electric power. So are all of the coaxial 7.62mm MGs fitted to British AFVs.