Ken,
I guess that didn't come accross how I wanted to relay it. What I meant was, like you pointed out, it wouldn't be a problem in EVERY case with chambers that would clean up, but I wouldn't want to be around any with chambers still left after boring and threading for a plug. I don't know the exact dimensions of the bore, grooves, chamber, and throat of the H&R .45 colt, but it is possible that it may clean up for one of the breech plugs. The twist rate is 1:16, I would prefer something in the 1:20 range. The shotguns would need a breech plug about 3-1/2" long to clean up the chambers and forcing cones IMO. One other thing to remember, the 7/8" plug is shorter than the 5/8" plug.