Well I have owned my Encore for many years and have shot over 1000 rounds out of it and the breechplug looks new.

I would never use a drill bit for cleaning bigger than the spec on any touch hole nipple or breech plug and 1/8 of an inch is to big for anything other than a cannon.

Don't get me wrong I use them for cleaning but I only use bits smaller than the hole twisting back and forth in my hand loosely to break up crud. I then soak them to get rid of fowling. and then clean well.

Lee, I can only think that you may clean to aggressively? Not sure good luck PBH.