1. First, thanks for all of the confirming comments.
2. Yes, I knew that anybody can make almost any firearm, save the obvious NFA exceptions, for their own use. Have already made a crutch-stocked 12 gauge with a 3/4" pipe barrel (20") with a hand grip, inside a 1" pipe. The rear of the 1" pipe, with the firing pin, has a section of lead. That's so it weighs 8 pounds for mild recoil.
3. There are at least 3 manufacturers of adapters/inserts for those 26.5mm flare guns. The most popular are for .45 Colt., .410 & 12 gauge flares. I think the original factory package included those very inserts. People are selling the flare guns and factory inserts separately. One can buy them separately and then create a .45 colt firearm for their own use.
4. Left out many details when I questioned the recommended "smooth bore" ID. I want to be able to hit something and not throw tumbling lead. That ID would be reamed before:
a) I use my 10 ton hydraulic press to push a rifling button down the ID to achieve the desired 1:16 twist., &
b) Ream the chamber to .480 &
c) Heat, quench and then temper the 5/8" OD "barrel" Already planning one with a 3/4" OD barrel if the proof tests at my son's 3 acre woods show any problem with pressure.
5. Pressure. The walls of my SS Charter Arms Bulldog, in .45 Colt are as thin as .0685". My 5/8" OD steel, when reamed for the cartridge (.480) will leave a .0725 wall that will be pressed inside of additional support.
6. Would still appreciate any comments on the .451 or .4515 ID and my planned proof tests. Steps of 4, 6, & 8 grains of Red Dot or 700x pushing a Berry's plated .452, 250 grain bullet.
Peace, WLeoB