Thanks for the tips and link! I used 1/2” EMT and a friction fit, 30”.
It’s pretty unwieldy, the gun was a long one to begin with, but it sure does work as advertised!
It looks promising.
I have been thinking of trying a .22 magnum shot shell, as it has 52 grains of shot vs 31 grains for the .22 LR.
Haven't gotten to the point of having a magnum to play with, yet.
I suspect it will give fairly dense patterns to 15 yards. Having a 24" tube on a handgun means you still have an easily handled firearm.