Depending on the model and serial number, H&R will fit new barrels to your action. Their website has the info on which actions they will fit new barrels to, and the caliber options. I don't think they do custom requests, but 22-250 might be one of the options, don't know. I don't think a threaded muzzle will be an option from them, but most smiths should be able to do that as a custom job. You might be able to get a barrel fitted that would have the correct bore size and have someone rechamber it if there is enough meat around the chamber.....but there won't be any custom twist options. My recollection is it only costs around $100 to get a new barrel (including the fitting).