I would highly doubt the USMC used A-4's and certainly not A-3's. How about M-3 Greaseguns? I don't think I've ever seen a picture of a Marine with any of these guns.
Why would I ask? I am making some Marine figures for one of my O gauge (1/48) Troop Pullmans, using a (slightly underscale but visually compatible) set of 1/55 "Bolt Action" US Marine figures made in the UK, so their knowledge was slightly imperfect. The weapons and deuce gear are exquisite but they give you A-3's and what would be an A-4 with an Alaskan on it, and Greaseguns. But the BARs, M-1s, Thompsons, Winchester 97s, and .45s are really sweet. I've posed the figures in "administrative" poses, unarmed, such as a crap game. The weapons will go on the bunks.
The troop cars were made by Weaver and Atlas now makes them in an upgraded version fwiw.