I sorta followed the For Sale thread by SBoomer of his scattergun to Greg. Not thinking that For Sale was place to wring out all the "can you?'s" "have you's?" and "what if's" that are likely on such a topic I thought I would take it up here. 
I know there are some potential touchy areas about these guns but I am not trying to be provocative, just trying get some ideas based on others experience and/or research and calculation.
Here's a couple questions to start;
1. What is the actual thrust of a 12 gauge BP shell as compared to a 38/55 BP load assuming factory loads?
2. What is the effective retaining area of that cross pin compared to say the threaded shank of an 1894 Winchester?
Thanks,
Dennis
P.S. I do have some experience with these, but some of is probably that of a fool venturing where angels fear to tread.

I knew a guy back in the early 80s he had a 12 gauge one in good shape and had a threaded liner silver soldered inside and screwed on a 4570 barrel. He shot straight black and duplex with the old 500 grain  Lee bullet At several about early gong matches. He never had any trouble , not trying to recommend it. But he got away with it.

Thanks for starting this Dennis.  I really have no idea what I'll ever do with this setup, doubt I'd ever turn it into a 45-70 or even a 38-55, maybe someday if I ran out of projects I'd treat it about like a #2 Ballard and make it a quarter bore something - or less... 25-20?   It got my attention primarily because I don't have one in the stable.  I do have the small H&A casting from Rodney so this makes a Brace not that they are the same, just the mfg. 
If I recall and maybe someone will set me straight, early 12 gauge had pressure measured in LUP, but even the 38-55 black was measured in CUP and there's little to no correlation between the two?  My engineer is away but the moment acting against a cross pin vs. what's spread around the diameter of a set of threads - I say moment -because of the inertia caused by the twist of the rifling?  a shotgun is a smooth bore so it's only a linear force...?

More to Ponder,
Greg

First I need to make a confession. Back sometime in the mid '90's a friend brought me one of those H&A 12ga. shotguns and a take off Shiloh .40/65 barrel. Told me to fit the barrel to the shotgun. I'm sure I raised one if not both eyebrows at the time. "What if it blows?" "Don't worry, I only have $150 in the whole pile", he said.
So I got after it. The hole wasn't straight to the breechblock face or the front of the receiver. So I got the thing in the mill true to the breechblock and bored the hole just enough to clean it up round and faced the receiver. Then I shanked the barrel to fit the receiver and give me snug headspace. This particular specimen had a set screw from the bottom rather than the cross bolt so I put that set screw divot in about 5 or 10 thou off in the direction it would want to pull it together when tightened. I had a pump repair business at the time and some real good industrial sleeve retainer grade Loktite which I slopped on barrel, receiver and set screw and assembled everything TIGHT with capital letters. I checked headspace with a feeler gauge, let it set a few days and tried a heavy BP load with I think a 400 grain bullet. I put on every piece of welding leather I could get into and turned my head like I was sure it would blow when I pulled the trigger. Bang. It was still together and headspace didn't move. He shot it for years in the BP schuetzen matches at Butte. The headspace never did move. Last I heard he still has it but as a backup gun now. 
Now I have a Baystate which I think is the midsize frame. At least it is bigger than my 3922 H&A. It is a .32 Long with a nifty little plate sliding sideways in a dovetail to switch from RF to CF. This one has the cross bolt which is tapered so it really does snug the barrel in without slop. I am thinking about fitting a .38 Douglas barrel to it and making the chamber .38/40 Remington. Not real sure why, except like my old friend, I'm into the stuff for a really low dollar amount. But all I ever hear now days is negative about these guns and maybe I'm not as bold as I was years ago. I really think this will hold up, I have seen several Baystate's in .32/40 and .38/55 with the cross bolt over the years on the net but don't know if they have a smooth  or threaded shank. Mine is smooth and I don't want to glue this one.

"GT, Funny you bring up the moment thing. Out of what must have been pure boredom yesterday, I started to calculate that exact comparison. I got sidetracked after talking to Gail about a set trigger mod on my Stevens. I never did finish the calcs. I got as far as starting to calculate the shear in the engagement of the tapered crosspin based on lowly 1018. Gut feel says the rotational moment is a couple pounds at most and a complete non-factor."

Mike, 
My preliminary after I mentioned it did amount to just a few pound of of force, insignificant.  BUT, I had a situation in the industry a few years back that surprised me... It's something I always consider now.  In brief, we use to make a cannon that a customer used for removing jewelry pins on a fairly large pieces of equipment.  (for those not exposed to this, the rigging on dragline's - chains, yokes, pins etc. is referred to as jewely, even though the links of this little chain may have an 6" cross section  and the little pins are 10" diameter)  Anyway, before they deemed this process unsafe, we'd make a cannon (18 or 24" od x 5' long) that would shoot a 5" dia. 8" long bullet propelled by det cord.  The back up for the cannon would be the blade of a D10 or D11 dozer and some serious pressure would be applied to hold the cannon in place, touch things off and the pin is removed from it's place in milliseconds.   First couple of acts - smiles were everywhere, resembling kids in a candy store... Now, with all this pressure holding the cannon in place, we'd occasionally see rotation of the cannon upon firing, occasionally as much as 10° - explain that???  it was determined that when we were making the cannon and backed the spade drill out of the hole once to depth, (usually 36") it produced a significant "rifling" and for the first few firings created a "moment of Inertia".  The original fit of the bore to projectile was loose - relatively speaking - .010"  -made a lasting impression on me 
Greg

The direction of wrap may have had something to do with it, but the rotation stopped as the bore eroded - if I recall it correctly.  Who's to say though, things I think I recall clearly aren't exactly the way others tell me they were...

scharfe wrote on Apr 16^th, 2020 at 2:23pm:


believe me, when you work around this stuff day to day, you look forward to the little things that aren't life threatening on the days off, so 4th of July - sparklers, smoke bombs, lady fingers...  22 rf, quarter bore BP...  

Well, I have reconsidered my .38/40 Rem. idea and am thinking when I get around to reworking my rifle I will make it into a .38 L. Heel bullets and all. At this point in time I think a plinker would get more use and provide more enjoyment.