First, thanks for the replies!
I've no interest in doing a smokeless conversion, just getting a proper BP setup. What I have is the barrel, forearm, breach plug and ramrod, no receiver, etc. I am 99% certain the barrel assembly didn't leave the factory as I found it because it has a factory H&R rifle (short) ramrod and the breach plug had some type of thread/floss (it looked like dental floss) wrapped in the o-ring groove under what appeared to be the wrong size of o-ring. That said, for those that have the press-in plug, is your barrel also threaded? As to the fitment on other receivers, I have a fair amount of experience with the (non-MLing) H&R single-shot guns, both rifle and shotgun, from the early "Topper" guns to the H&R/NEF SB2 Handis. I have several H&R/NEF receivers as well as numerous barrels and most interchange, although there are a couple of combinations that are either a bit loose or no-go, so I am good as far as properly fitting on at least one receiver I own. (NOTE - FOR ANYONE READING THIS, NEVER FIRE A BARREL ON A RECEIVER NOT DESIGNED FOR THAT LOADING, i.e., do not fire a .35 Whelen on a older Topper/158/etc. receiver even if it seems to fit.)
As to firing the gun with a press-in plug, I'd rather not in case of a hang/mis-fire. If I were an experienced ML/BP shooter, I might feel differently. Also, I reload both shot shell and metallic, so I have 209s, dies and brass for .25 ACP and plenty of primers already on-hand, so a conversion to one or the other seems to make sense from that standpoint. Not that buying caps would be a big deal, but I also understand, perhaps incorrectly, that either 209 or .25 ACP are surer sources of consistent ignition as well. Again, as I understand it, and perhaps incorrectly, that the 209 conversion is a nipple replacement (which would require finding a screw-in plug) or a complete new, non-factory breach plug and the .25 ACP conversion is a new, non-factory complete plug. My initial thought is to simply go with whatever turns up in the next week or two, but if anyone has any suggestions, I'd appreciate hearing them.