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Calibers on the menu are .22 Short (a small cylinder w/ matching frame that accepts only Shorts), .22 LR (you can also shoot .22 Shorts), .22 WMR aka .22 Magnum (can be had with a second cylinder for .22 LR/Short), and .22 cap and ball. Even the biggest of the Minis is still pretty darn mini, though.

yes, should shoot fine for the 20'

Yes, the only side effect would be less accuracy as the bullet literally has to "jump" before it engage the barrel rifling.

Yes, the shorts are fine in the LR model.