H&R will do it if you send it in to them. When you order one or have it sent back to have a barrel fitted it is an option, or at least used to be. From what I hear they do a pretty good job for not much money.

I am pretty sure a decent gunsmith could do it as well. I can't see it being a complicated mechanism. A gunsmith can fit the barrels as well I believe, if you don't want to send the whole thing back to H&R.

Paul