Hello John,
From all I have read, airgun barrels are softer than powder burner barrels, this may add to the cost of re-boring, if it can be done ! and then there is the crowning and chamber fees.
Now you can buy barrels in other calibers for air guns, I have read that James Maccari had some for sale, but I can't remember what calibres. And Rich in Mich has .20 caliber air gun barrels, for the CFX or Shadow/Hunter Series for $75.00. I don't know if CharlieDaTuna does this type of work, drop him an e-mail and see. I would rather have it done properly the first time, than need to send it out after I messed up.
I believe Gamo sells replacment barrels, and we know Crosman does.
Hope this helps some.
Bill