Caliber conversion tubes/liners/sleeves in a laminate antique

[svt-40]

Was a BP shotgun action strong enough to fire 45/70 from a 18" conversion tube?
This is Janssen, Sons & Co. 12 ga 2 3/4" sxs with laminate steel barrels.

 

[svt-40]

 

[mooncoon]

the rule of thumb that I have used for any conversion was to multiply the original operating pressure times the base diameter squared of the original shell and compare that number with the same figures for the new caliber. A black powder 12 guage would be 4000 psi x .8 squared (at a guess) or about 2650 vs a 45-70 at say 25000 x .5 squared or about 6250. Even allowing for low pressure smokeless loads in the 12 guage or say 7-8000 a black powder loaded 45-70 is in the upper range of what the action was designed for.
I think the answer is that it can be done but you are in the upper limits of what the action will accept repeatedly over a long period of time and you do not want to use heavy smokeless loads in the gun.

cheers mooncoon

 

[oldstarfire]

Unfortunately, Janssen was a brand of really cheap "hardware store" guns made by a Belgian maker - most at shows are trash - I would not consider making anything out of a Janssen except a fence post or maybe a Toyota part.
Just my opinion.