I built one for a trunk gun, but was wondering the same thing...is it effective?
The short barrel is ideal for autos for obvious reasons, without the stock, you have to re-consider things beyond the spray and pray modes.
So, take a sling and run it under your strong side and over your weak side shoulder. Adjust it so that when you push out on the gun, it stops just right for you to get eyes on the sights. Use those uppper body muscles and stablize it. Tuck your arms in and find the right body geometry for you. I also run my forward hand back into the mag a little to help secure it all.
I can hit pretty well out to about 60 yards. I can aquire the target very fast, but the recoil (in 7.62...and DON'T go building the other caliber into pistolas please!!) is harsh enough to send the muzzle off target...so you just deal with slower second shots.
I think it's tactically viable with practice, and a very cheap way to put a non Class 3 short barrel rifle caliber in your truck that costs less than $300 or so...
As a pistol and if you have a CHL, it allows you to take a "short rifle" with you under your CHL rights....