I use the Element M4 hopup unit with a Prometheus rubber a long with a Madbull tight bore barrel.
Got the unit from CapitalAirsoft and the other bits at Airsoftparts.
Eventually I am going to swap things out and get a Shredders concave spacer in there, but for now that combination works great.

Glad you enjoy it!
Maybe you should get some German kit and come out with the cool kids ;)

What length barrel did you use? I haven't actually measured my barrel yet but I read somewhere that it is 450mm.
As for German kit, I am sporting some surplus flecktarn cammo, but I don't have any vintage style gear yet. If I can find some magazines somewhere (every place I look they seem to be sold out...) I will get reproduction mag pouches for sure. There is a place I can order a flecktarn field cap that looks alright, but I'm not sure reenactors would let me pass with post war gear.

If I did this right I should have pics of my MP44, close up of rifle but (with repaired hole due to shearing off screw and sending it out the side) and cammo.