Your friend was certainly an exception.
Despite many "war stories", I have heard many times from GI's who brought back Mausers that hey actually got them from depots and dumps just before they shipped back to the States. Even "non-fighters" like Ordnance, etc., did not have unlimited space for transporting souvenir rifles, and had the same rules as other GI's (often one rifle and/or one handgun per soldier) depending on local policy. (RHIP, of course!) Some commanders banned souvenir handguns, one saying that he lost more men to accidents with such unfamiliar pistols than he did in combat!