I have the answer you are looking for.
Basicly the through bolt with the flash hole, for the bottom barrel that comes with the gun is very soft, and rusts quickly, so you really don't want to modify it. Take it out and put it in the box for some other day.
Now dig in your bolt box for a #8(hardness) bolt with the right threads, and at least as much thread as the original plug, I belive it will be 10mm X 1.50 threads 1 1/8th inch long.
Drill a #16 hole starting from the threaded end 1 1/4" deep into the center of the bolt.
Chamfer the end of the bolt, and remove the first couple of threads to match the original plug. now screw the bolt into the breach plugs, until very snug, take a long steel rod (Ready rod) sharpened to a point, and drop it down the bottom barrel, you will feel it going into the breach plug face hole, now strike the rod, and move around a little, this will mark where to drill the flash hole, remove bolt, and drill flash hole into side, remove bottom breach plug and screw bolt into it so your hole matches up, now chamfer the hole to match the breach plug, remove bolt and replace the lower breach plug. cut the flash plug off the bolt, aprox 1 1/4" long, and cut a slot so you can use a flat screwdriver to install and remove the plug.
Last thing to do, is to screw the plug into the gun, and tap the open end of the plug with a #12 X 24 tap, aprox. 1/4" in from the bottom. (this will be your cleaning hole) coat the outside threads with pipe dope, and install the flash plug into the breach plugs, and install an allen head #12 X 24 x 1/4" plug into bottom. file bolt flush with lower breach plug, and you are done!!!
It takes some time, BUT it does work!!
Let me know if you need anymore info.
Scott