I think it was the novelisation of Aliens that clarified Hicks' shotgun was a family heirloom handed down through the generations on his father's side - it wasn't an officially issued piece of kit, just something he brought along himself. Kinda like soldiers taking their own sidearms along in World War 2.
As for the VP70, that's an obscure enough piece of kit it's at least somewhat plausible to pass it off as a future gun (especially given it's futuristic-looking design). The Uzi is iconic and very much grounded in the last few decades, making it harder to flub in the same manner.