It’s pretty clear (to me, any way) that Vader’s helmet draws from both design sources. As for the firearms, Lucas used *tons* of hacked surplus weapons – there are a couple of scenes in Episode IV where you can see shell casings from blanks falling away from Storm Trooper blasters as they fire. They were using modified Sterling SMGs, with the blaster effects painted in by hand afterward; the effects team either missed the casings or didn’t think they could cover them up without otherwise disrupting the background.
(Sorry about taking so long to approve this comment! I must have missed the notification!)