My thinking is they are pretty much useless. To big and heavy for a holster to carry as a backup to a rifle. On my useful scale they rate below a pistol.

For the same money you can get a carbine (or asshalt rifle), many not as accurate as a full sized rifle, but more accurate than most pistols with a longer useful range.

Spray and pray noise makers have a limited utility IMO.

I had a chance to buy a pre 1944 M-3 grease gun for a few hundred dollars and passed. A fairly high novelty factor (that would likely have worn off fast), but I'd shot them before and found them to be generally useless.