Why would a single shot only be capable of firing 22lr?

I'm not a Rossi company engineer but my guess is that you could experience a build up of lead or powder fouling in the chamber and have trouble chambering a LR round next time you try.

It likely would be safe to shoot sub caliber rounds (ie. .22 short, .22 long) out of a single shot or bolt action .22LR rifle. It's generally not recommended because if you do it enough, you will get lead fouling in the chamber where it's not covered up by the cartridge case walls.

A small number of sub caliber cartridges won't be a problem, but if you do it enough, it can make it harder to seat fresh cartridges (due to build up near the front of the chamber) when you insert a .22LR cartridge. Likewise, the lead buildup may hold up spent casings, making it harder to extract. Both situations may lead you to believe your rifle requires repair rather than just a thorough cleaning with a brass brush. People usually don't scrub out the chamber of a rifle with a brass brush on normal cleaning.

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On some models of rifles, if the chamber is separate from the barrel, it may be made of a softer steel than the barrel, and might not be intended to handle the heat from the burning powder. Again, one or two sub caliber cartridges won't be a big deal, but it may cause chamber erosion after a while. It'd take a long, long time before it caused enough damage to the chamber to become dangerous or even affect functioning, I guess.

That's about the only two things I can think of but if the manufacturer says not to do it, it's basically a "do at your own risk" sort of thing.

The manual did not say the firearm was " incapable" of firing the shorter than extra long rounds. Just don`t do it whether or not it is Rossi`s CYA clause. The .22 round uses a heeled bullet which the bullet diameter is within a couple thousandths of the neck diameter. The residue or lead sheer will not be as drastic as larger rimfire rounds because of being a heeled bullet but residue will still accumulate after long periods of use. Many people have fired shorter rounds in a longer chamber but those people, like myself, usually are not depending on a warranty clause that could be voided by not following warnings in the manual.

I`m not saying do it....just saying it will not blow your rifle up if you choose to do so.

Due to a build up of lead/fouling same thing happens when you fire lots of .38 Specials in a gun chambered for .357 Magnum. My model 66 in notorious for that.

Sounds like legal boiler plate.

Shorts will leave a carbon ring and make the chamber short if not cleaned well.

Partly due to the answers you got and partly because 22 short is nearing extinction...

Why don't you try it and find out? Maybe the engineers that designed it know something you don't.

maybe the extractor doesn't handle them well, either way, follow the manual.