Not a Giffard, suspiciously like a Greener though

[Benelli B76]

The experience of the Volkssturm suggests that untrained personnel with rubbish weapons are at most a minor irritant to a professional army. Their most effective weapon was not even a gun - the Panzerfaust.

If you need to arm folks quickly with a vaguely useful weapon, in the absence of an established mass-production arms industry, you want something like a Sten. Witness the various SMGs turned out during the Yugoslav civil wars of the 1990s.

AKs would be better.

As you say an SMG that fires from an open bolt is the easiest to produce from a bit of pipe a couple of springs and sheet metal. The mag is the probably the most complicated part to get right. That is why most Sten MK 111's were made by Tri-ang Toys ( Lines Brothers) in London. You do not need a lot of complicated production tooling as for a standard rifle. After the war they were mostly destroyed.

Baz