I reload non-tox shells for my 104 year old Rem 1894 in 10 ga. So far I've tried Bismuth, Nice Shot and ITX. My Remington has damascas barrels and the original full chokes. I've not noticed damage from any of those types of shot. I've got a few thoughts on some of the comments made above.
While pressure is a valid concern, recoil from loads that are heavier that originally intended probably cause more problems in the long run. 100 year old wood, in a gun that has stood upright in a closet or gun cabinet or where ever, probably has had some measure of oil soak into the head of the stock, weakening it. That's where you get your first stock damage. Side lock guns and butt stocks that don't have tangs that slip inside the action of a boxlock will crack before those stocks that do have a tang that is inside the rear of the boxlock. My 80+ year old Iver Johnson Hercules has tangs and I expect it will be a viable shotgun for many years to come.
The series of article Sherman Bell did in the Double Gun Journal should be required reading for anybody contemplating using an old double for waterfowl. His reccomendations on loads, all pressure tested by him with a calibrated test gun, work quite nicely on waterfowl. I've been using them on late season whistlers here on Champlain for 2 seasons now. The best place to find the complete list is on the Parker Forum under the reloading section. Be advised though that the 2 best smokeless powders for use in these old guns, IMR 4756 and IMR 7625 went out of production last year. Reasonable alternatives (in pressure tested loads) would be Blue Dot and maybe 800X. Believe it or not, black powder, in heavy loads, give modern levels of velocity with moderate pressure levels. Yes, its a pain to clean, BUT, the results are pretty interesting. Use fiber wads if possible as the black powder has a tendency to leave plastic crud in the barrel from having melted the bottom of a plastic wad. You can still buy Federal once fired paper hulls from BPI and others. If you use plastic hulls (especially with smokeless powder) consider using new primed plastic hulls. Fiocchi and Cheddite hulls are straight interior walled hulls and have copious volume, which helps keep down pressures. Lastly, if you do use new hulls, roll crimp them. Most times, and with most loads, roll crimping gives pressure readings 500psi less on identical loads. If you are trying keep loads below 8000 psi, as I am, that's significant.
The DoubleGunshop.com website,
http://www.doublegunshop.com/forums/ubbthreads.php, is a plethora of info on loading and shooting these old guns. Anything written by Drew Hause or the guy who goes by the name Researcher, is worth reading. Lots of good info there, especially on American doubles.
Lots of people will tell you that these old guns are unsafe to shoot. Maybe yes, maybe no. Each gun should be considered on its own merits. Having a garden variety gunsmith examine it for structural soundness may not be the route to take in determining so. Find a smith with old gun experience. I've had several smiths tell me that no gun with damascas barrels are safe to shoot ever. And that's just not accurate.
John Bourbon