First of all, please ignore Bob Taylor's comment, as it was a weak attempt at humor, something that he employs from time to time on this message board.
This topic has come up on the message board in the past. As I recall the consensus among the Armed Guard veterans who contributed to the discussion was that Armed Guard officers were issued their own side arms (i.e., revolvers) and that while on security detail at pier-side or at anchor, Armed Guard sailors would be issued either .45 or .38 revolvers, or rifles, for the duration of the watch. (Rifles were also sometimes used at sea to fire at floating mines, according to some recollections.) There was never a mention of submachine guns so far as I recall.
Keep in mind that the primary purpose of the Armed Guard was to provide gunnery protection for merchant ships at sea. Most Armed Guard sailors were trained to served as gunners, commonly using 3-inch and 5-inch guns and 20mm. mounted machine guns. The Armed Guard sailors were really not "guards" in the sense of a security force guarding ships in port, although, yes, they did stand gangway and anchor watches on their own ships. There would have been relatively little need of or opportunity to use submachine guns.
Any Armed Guard veterans in the sound of my voice, please correct me if necessary.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website