My experimentation with breech plugs for break action inline muzzleloaders using cartridges was triggered by articles about conversions of factory breech plugs to use .25 ACP cartridges or Variflame adapters in place of #209 shotgun primers, on the theory that less powerful primers give better accuracy. My reasoning was that, if one were to use a primed cartridge case to set off a muzzle loader charge, why alter a factory breech plug to accept a cartridge case? Why not build a breech plug around a cartridge case from scratch? To answer the objections of so-called traditionalists, Harry Pope was building rifles this way in the late 1800's. To this end, I began examining the factory plugs. I found that CVA break actions, both the older recessed breech plugs and the newer Quick Release types, and Rossi Muzzle Loaders, all use a breech plug with a 5/8 - 18 basis. Searching on line I found that Home Depot sells a line of Crown Bolts, Grade 8, that are 5/8", 18 threads per inch. So far, I have built proof-of-concept breech plugs for both the older and newer CVA's and the Rossi, in .32 ACP, .38 Special, 9mm Luger and 5.56 NATO (cut off to 1') primed cases, and they have all functioned well with loose Black MZ. I will be posting pictures of my breech plugs when I get around to taking some, and providing information about ballistics as I assemble it. Right now, I am at the "well, it does go BANG!" stage.