Cheapest method: remove bbl and have it threaded by anyone in town with an engine lathe. If they're a professional machinist they ought to do a fine job. Cost is an hour on the lathe, a flash suppressor, and your time in removing and reinstalling the barrel. (Armorer's wrench, bench vise, and a hammer and punches will be required, and a receiver block is highly recommended. If you don't already have these, it's not the cheapest method.)
Not as cheap method: remove bbl and ship it to Bushmaster. Reinstall new barrel. If it's a Bushmaster upper and bolt it
ought to headspace properly but you really need headspacing gauges to do this properly. (Again, if you don't already have the tools to replace a barrel and gauges, it's not that cheap.)
Slightly less cheap (but easiest) method: ship the whole upper to Bushmaster with the instructions to replace the bbl with a threaded one. You're out shipping and the cost of a new bbl, and if the old bbl already had a few thousand rounds on it...well, you were going to have to do it eventually anyway, right? I'd think this would be the way to go. Give Bushmaster a ring to see what the total damage would be.
Very expensive method: Buy a new completed upper.
U.S. government expensive method: Buy a new AR.
