spm1138 said:
Nooooo. You can't add bits from the MP5 to the Thompson.
Go to HKPro and look at the early prototype pictures.
It looks
really WWII German.
That's because it IS WWII German. The guys who founded HK via the CETME project in Spain took the StG44 as their prototype for the G3, then took what was the firing mechanism for the MG42 (roller locking), developed it further for a rifle, and made the CETME Rifle, which was further refined under HK to become the G3. Ergo, the G3 is a direct result of nazi weapons research.
I honestly think if WWII was still going today, the timeline of weapons wouldn't be that different, just accelerated. Polymer would've come around as a cheap and easy alternative to stamped metal, first replacing wooden parts, then eventually replacing entire frames, leaving the working parts and parts under stress the only metal left. The only difference is that this would've come about earlier. Some time around the mid 50s, say.
The weapons we have these days are the result of a war, in the end (The Cold War), so it really wouldn't be much different. When you design, just keep in mind the trends of various countries. The Nazis and the Americans both love the concept of a technological advantage, and they usually pioneer things (Eg. Garand, G43, StG44). The Soviets were the masters of mass producing robust and functional weapons that keep working way longer than they should while costing way less than they should (SVT40, PPS, PPSh-41, T-34), and the British made quality weapons, sometimes a little obsolete, but usually quite functional. The Sten and Bren, for instance, are pure quality, but the Bren was outclassed by the German machine guns, and the MP40/Sten debate could go on for eons.
Keep in mind those trends, and the common idea of (usually) building upon weapons that already exist, and doing things dirt cheap, and you should be fine. My PM box and small library of firearms books are always open, too