It sounds like you have a Remington No.9 Rider Single Shot (The No.3 Rider didn't have the patent date marked in it).
If it is, it should have a removeable hinge pin, like the one below.
Please consider that the gun was made with steels, of 110-year-old technology, and not the same as modern steels.
Also consider that the ammo companies upgraded ALL commercial shotshell ammo about 85 years ago (in 1925), with higher pressures, etc.
Then think about exactly what stress' that metal's been through in the last 110+ years.
IMHO, before firing it, it would behoove you to take it to a gunsmith that knows what they're doing, and not some AR parts-changer, to get it checked out for internal tightness, on/off face (like headspace), and small dents (fixable) or bulges (an unfixable time bomb), etc, etc.
THEN, if/when it checks out OK, special, low-powered modern ammo for these old guns (I have an Ithaca from 1911) is available from the likes of RST, PolyWad, Kent Cartridge, GameBore & others.
NO other modern ammo is safe - including so-called "low brass", and "target" or "field" loads.