Musket Cap Breech Plug

[eod20]

looking for opinions on the safety of this idea      i want to drill out and thread my 5/8 inch 50 cal breech plug to accept a 1/4x28 musket nipple     it is now a standard 5/8 inch factory 209 primer breech plug   

[Hammerspur]

Strikes me (pun intended ) you may have a problem when you smack that tophat with the small firing pin intended for centerfire ignition.
 
                                                                 
Other more informed readers may know for certain, but I suspect you may get perforations of the primer which would probably allow fouling blowback into the standing breech and firing pin channel.
Outside of this messiness I don't imagine any unsafe conditions would present.

The old (70's era) Huntsman's had a fat firing pin intended for percussion cap strikes but they also work just fine on 209's.

[Busta]

I did a conversion for #11's, and you might want to read this before proceeding. I think you will have problems with the firing pin also. I would suspect you might even have trouble setting off the cap with the larger hole in the nipple. It will set off #11's, but after about 5 shots your firing pin will need some serious attention. The older Huntsman firing pins had a wide firing pin like Hammerspur mentioned, they would be far bettter suited for musket caps.

My 4th post in this thread has a pic of my #11 conversion.

http://www.gboreloaded.com/forums/index.php/topic,124756.0.html