Here's my Ithaca side by side I traded into last year. I cut it down to 19" right after I got it with a hand held band saw I borrowed. Made a real nice clean cut...that was crooked as hell. I spent several hours with a file working it down closer to square.
Anyway...in a more perfect non NFA world I would cut it again but about 2" past the forearm and take about 2" off the stock.
So confused. If you were going to NFA a shockwave, why not just put a pistol grip with a folding stock? Same short package for carrying and shooting from the hip, but if you want to aim you can just deploy the stock. Why mess around with using forward pressure on a sling for stability? That seems like something to do if you don't want to deal with the NFA hassle.