think so
Yeah, I think lopping the tube will leave the gun mostly a short range hunting proposition. Like maybe 20 yds at best. Yeah, no doubt you can kill a squirrel, sitting, in the open, on the ground, past that. But.......? We have some 12 ga Cylinder choked guns show up at our 3 gun matches, and much past 20 yds, their ability to consistently knock down plates and break stationary clays is pretty apparent. I'd think a 20 ga, with its smaller payload, will be more of the same. Squirrels up in the trees, protected by leaves and limbs, will need more choke. How much more is debateable. For a GP, do all gun, modified choke seems about right.
A 26" barreled single shot is going to be pretty handy and short to begin with, much shorter than any repeater, due to the lack of any receiver that would house the action/bolt.
I'd leave it alone. Wouldn't want a "sawed off gun" for self defense either, but that's another post.