Just unsolder it and pull it off. Really. If it is only held by solder, it should come off. I am not an M1 carbine mechanic, but I have done a fair amount of soldering and desoldering over the years. I would recommend getting some heat control paste (Midway has it) and applying it to the bore in the area of of the flash hider. It won't hurt to put some on the first few inches of the barrel exterior behind the flash hider, either. Apply just enough heat with your propane torch (a good hi-temp heat gun will work, too) to get it loose. You won't hurt the barrel a bit as long as you don't go crazy with the torch.
Two gunsmiths have not been able to get it off? Are you sure there is not a little set screw or pin that is hiding? Surely Bubba didn't thread it and sweat it in place?
If your really serious use a dremel or other cut off tool to cut the majority of the flash hider off. This will give you a look at the end of the barrel to see if it was threaded. you could also grind two flats on opposite sides of the stub after you cut it off. This would give you a place to use a opened wrench to twist the Flashhider off after you heat it.
People get Soft solder and Silver solder confused.
Soft solder typically melts around 400 to 500 degrees.
There is a silver BEARING soft solder that has about 3% silver to prevent tarnishing, and many people THINK this is silver solder.
REAL silver solder is actually silver braze, and it melts at around 1100 to 1300 degrees.
If the brake was soft soldered, I doubt it would have stayed put.
If it was silver brazed (silver "soldered") you won't get it off with a propane torch.
A propane torch simply doesn't put out enough heat to get it to melt.
If you fill the barrel with Heat Stop paste to protect the bore, and use a HOT torch like a Turbo-Torch you can likely get it off without damaging the barrel.
The trick is to heat the barrel and brake as fast as possible to limit heat spread.
Just quickly heat the barrel and brake to a red heat, then grab the end of the brake with a Vise-Grip and turn it off.
Allow the barrel to cool on it's own.
However, the red heat needed to melt the braze will ruin the finish.
As above, I'd cut the brake off, and then use a small hacksaw or a jeweler's saw and a Dremel tool with cut-off wheels to carefully remove the remainder.
One trick is to cut off the brake, then use a cut-off in a Dremel to carefully split the remainder down almost to the barrel.
Then use a light hammer and a dull chisel to split the remaining portion.
Usually, when you split it, you can "peel" the rest off the barrel.
Pretty risky as you can easily cut into the barrel withought knowing it and leave a permanent mark. I'd try acetylene, and be sure you have a way of getting a good grip on the flash hider, and have the barrel/rifle clamped down so you can knock it off with a hammer when the solder softens enough. Anything up to a dull red head shouldn't bother the barrel as it isn't heat treated, just let it air cool.
BTW, that silver bearing solder, about 5% or less is used in the better non lead solder for plumbing to make it flow much easier, not for non-tarnishing. I hate the cheap no-lead stuff and always use the silver bearing solder on any soft solder non-electronic job.
If you know anyone that has a vertical mill, you could machine a full length slot with some precision control over the depth. Another method would require a lathe - just turn down the flash hider (actually, it looks more like a muzzle brake) until it isn't there anymore!
If you ever visit the Knoxville, Tn. area, stop by and I'm sure we can get it off.
As valuable as M1 carbines are I would turn the barrel in a lathe and cut the hider off. I would not heat the barrel to 1100 degrees. Centered correctly in a lathe you can skim right down to the original finish. Just my two cents. riceone.
I would opt for the machine/lathe method. Least likely to damage the barrel without grinding, heating, hammering, etc. Any good machine shop would likely do it at the hourly rate since it would require very little set up.
That front sight is behind the flash hider - it wouldn't be a problem. You would use a dead center in the end of the hider with a 3-jaw chuck (if the jaws are long enough.) Otherwise, use a dead center and face plate with a "dog." (You hold the portion of the flash hider that is in front of the barrel. You only have to turn down the portion of the hider where it is on the barrel. Selection of the proper tool bit will allow it to be turned right up to the face of the front sight. The receiver end will need to be supported by the lathe tail stock. I don't remember what the receiver end of a carbine looks like - but it shouldn't be to much of a problem to rig up a fixture if necessary. I suspect your gunsmith didn't feel like bothering with it.
Oh, it just occurred to me that you could drive the rear end of the receiver with the head stock and stick the tail stock center in the bore of the flash hider. This would also allow you to reset the tail stock in the barrel bore (after the major portion of the flash hider is removed) to allow some fine "cleaning-up" of the barrel area where it was mounted. Any "real" machinist should be able to do this (I'm not a real machinist - I just tinker.)