Whatever you do, don't shoot them with trap (target) loads or any other standard modern "smokeless" (nitro) powders. While light by overall weight of powder, some trap loads are hot, that is to say they're fast burning powder that develops peak pressure quickly.
There are low pressure shells made by Gamebore in the UK and distributed by Kent Cartridges in Canada (12, 16, & 20 ga.). They're based on nitro powders, but the powder selected is a coarse blained, slow-burning variant. I haven't seen them in stores for a couple of years, but Gamebore also has(d) black powder 12 ga shells. All of these shells are 2 1/2" to fit these nice old guns.
I love wing shooting with the old SXS hammer guns, 1897's, and the like. I've used Ben's idea with the tire many times. (I just lean the butt plate up against the back tire on my truck, nestle the forend on a bunch of cushions or something soft, aim safely and trigger from the other side of the truck with a rope.) A thorough inspection of the gun before and after test firing may detect bulges in the barrel or other abnormalities
before you hold the gun up to your face and pull the trigger.
However, our enthusiasm must always be framed within the bounds of safety. There are a few guns in my collection that I won't fire. To be honest, I don't expect catastrophic failure even from those, but I do expect that if I didn't hurt myself, I'd almost certainly hurt the gun.
Junk will end up back on the scrap heap in less than a generation, but fine guns outlive us all. As such, all we really pay for is the right to play with them and be their custodians.