You can take the Bushmaster short buffer tube, springs and components and screw them to a standard pistol lower, stick the Bushmaster bolt into a metal upper and are good to go.
The Bushmaster barrel can be easily removed and replaced and/or another barrel can be installed.
Have done it, but not shot it so be careful and check everything first. Check headspacing, mine looks good to go.
The barrel nut is smaller and and requires a spanner with two pins in it similar in configuration to the large AR spanner.
Coincidentally, two nights ago I found a spanner at REI in the bicycle dept that looks like it might work, it's a little on the filmsy side.
An alternative is to cut out a piece of aluminum plate and drill two holes in it for drill rod or even old drill bits at the appropriate spacing to make your own.
I happened to have a spanner for years that I never had any use for and it was perfect for loosening the barrel nut. The nut was fairly tight but didn't have to heat it, did use a DPMS or whatever upper receiver holding block.
DON'T heat carbon!
The gas block is easily removable, be sure to loosen the set screw ALL THE WAY!!!
Use a small gear puller and socket to pull it off if necessary. Watch for small parts.
You can machine an insert sleeve to adapt a common 3/4" gas block to a .625" barrel.
I'm putting a 10+ inch pencil barrel on mine [from J&T].
I don't understand the beef about the quick detach muzzle attach device. Never had any problem w/ it, but there's always the first time. Clarification?
Use a diamond "stone" to dress up the muzzle so the QC device goes on and off smoothly. Keep it clean.
The quick connect muzzle is .5" dia so it probably could be threaded to accept both screw on or QC attachments, leaving the existing detents.
Haven't done mine yet.
Bushmaster uses a lot of LockTite on the barrel nut and upper and you really have to carefully clean it off. Use a dental tool.
Sometimes the Bushmaster upper doesn't fit a metal lower so that the rear pin can be inserted.
I've found that the carbon "flash" from the molding process needs to be smoothed a little on both sides of the pin hole. A diamond stone does a nice job. DON'T mod the pin holes!
Barrels can be fluted on a common vertical mill using a a rotary index table and attached 3 jaw chuck and an end support. Use a radiused carbide cutter. A buddy does .50 cal barrels this way.
Hope this info is useful. Don't be intimidated by these Bushmaster parts.
Customizing these Bushmaster things is fun!
The spanner form REI fits the BM Carbon 15 barrel nut nicely and looks like it will work. Mine is already loose so couldn't put full force on it.
The brand is Park Tool, model SPA-1. $8.