PetahW pretty well nailed it.
The 1890 was chambered in 22 long, which was a LR case with a shorter, lighter bullet. I've got an 1890 so chambered that belonged to my great grandfather, originally purchased in 1927, if I'm not mistaken.
Ammo is not impossible to come by, but you often have to settle for .22 CB Long instead of the full power rounds. I will say that I haven't seen any in a little while, and given the rush on ammo, and I suspect that none of the major manufacturers will be devoting a production run to something relatively obscure for quite a while. In other words, I do buy everything I can when I get the opportunity.