If you are talking about the 50 caliber muzzle loader barrel for the Mossberg 12 gauge Shotgun model 500 or the Maverick 88 - you can't remove the breech plug. You have to clean the barrel like a flintlock.
I just bought one from Mossberg's web page (arrived yesterday 12/21/08). It didn't come with any instructions. Fortunately, I found the manuals online at the following addresses:
Muzzle Loader Barrel
http://www.mossbergs.com/manuals/Muzzleloader.pdf
12 Gauge Shotgun Model 500
http://pdf.textfiles.com/manuals/FIREARMS/mossberg_500.pdf
Good Luck
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