If I were in your position I would be tempted to try another barrel that is known to shoot your chosen projectile well. A good machinist should be able to set it up without spending too much. Once it is sorted you can probably ask a premium for the barrel you currently have since it has proven results. Trying anything else has no guarantee to work with the projectiles you desire and worse yet may ruin the great barrel you have now.
Just 2 cents.
Your 2 cents is likely how this is going to play out.
Even though I'm not brave enough to go for it, I'm curious to see what would happen. The math shows that there would still be some rifling left.
Nothing to scale there, just using the bore and groove diameters listed by LW and the sized pellet as an approximation to the size of the choke. By bringing that 0.211 choke down to maybe 0.214 (removal of 0.003) there should still be about 0.003 inches of land in that choke section. That's still deeper than polygonal rifling. And removal could be stopped to still have a very slight choke if done slowly and carefully by pushing pellets through it lots of times during the process, would also need to polish up the roughness before each how much have I removed measurement, otherwise the roughness could be mistaken for tightness (ie leave the "choke" at 0.214 and the rest of the bore at 0.215).
As has been pointed out, with as good as this barrel is with the 18.1s, I think I've talked myself out of possibly ruining a good barrel. Still curious what effect a gentle transition from 0.006 deep rifling into 0.003deep rifling, with an overall slight choke (from 0.215 to 0.213?) would have on accuracy.
The whole point of using a cast-in-the-bore lead lap is that it removes material from the grooves AND lands evenly.
So your rifling wouldn't be shaved away, the entire bore itself would get bigger when done properly.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://airgunwarriors.com/resources/library/Hand_Lapping_An_Airgun_Bore.pdf&ved=2ahUKEwinq7ebz4rsAhW9kHIEHdlbBK8QFjAAegQICxAC&usg=AOvVaw2gaDqUBXqrWYXdqw_UN
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