UB,
The magazine extension is PARALLEL with the bore, not in front of the muzzle. Shot won't spread far enough to begin to impinge on a magazine extension until it gets several FEET out in front of the muzzle. And the muzzle blast may leave some smoky deposits on the extension but certainly won't hurt it, unless maybe it's an ATI plastic one. Lots of the 3-gun shotgunners run extra long mag extensions that protrude in front of the muzzle by several inches with no problems.
I would definitely be sure and use a barrel clamp on it though- not because the extension isn't strong enough to take a bump, but the magazine tube threads it screws onto might not be.
lpl