beleg, you are on to something.

Sarg, why not commence cutting about one inch at a time, stopping only long enough to test the regulation? Lopping off too much might be permanently problematic.

Maybe if you started with a selct load, you could lop-and-shoot until the gun shot into the load?

FWIW, and somewhat off topic, but years ago I had a lot of fun with a cheap single-shot shotgun, patterning and chopping. I didn't care about final performance, and was only concerned about what I could learn along the way. I used the gun for grouse hunting and made a sling for it so I took it along while elk and deer hunting. Curiously, once the choke was removed, patterns stayed essentially the same from 28 inches all the way down to 20. Then I removed about an inch, bringing it down to 19 and the result was bizarre. Patterns with all the cheap target loads I had been using along the way not only opened up but showed dramatic clumping, making for really poor performance. I actually got rid of the thing after that.

I might add, that a 19-inch barrel, 4 lb 12 gauge gets your attention when you fire a 1 oz rifled slug thru it and even more so when you touch off a round of 3-inch Mag 00 buck! YEESH!!

Anyhow, all that to say, don't take off too much at one time. Go slow and maybe you will walk the gun right into regulation. I suspect the first or second chop will tell you if you are heading in the right direction.