Say you had a barrel, already fitted to an action, and you wanted to lap a slight choke into the barrel, how would you go about it. It would be a barrel for lead bullets, like a rimfire or air rifle. Would it even be worth the trouble to try?

You might try it as a last resort before scrapping a barrel.

Mark the barrel where you want the "choke" to begin, pull the barrel, pour the lap and work from the breech, stopping short of your mark. You can progressively approach the mark every few passes to provide a smoother transition. Check your progress by slugging. Good luck!

Fire lap the barrel. It will naturally get a choked affect.
I've done this a couple of times with bad fouling barrels and they not only clean super easy they are deadly accurate with lead bullets.

What Rusty said.
Regards, Joe

Hadn't though of fire lapping, that might be the ticket. Have the stuff for that on hand also. Not sure how that would work in an air rifle using pellets, they don't have much bearing area to hold abrasive.