On a 77 to change caliber you would have to change the barrel and the air cylinder ,the piston moves in.They are made as on piece .It would be possible to change just the barrel I guess but that would be hard to do and expensive ,just buy another gun would be better.Also if you did get a cylinder and barrel of your choise ,you must get the correct cylinder size old cylinders are a different size to the new cylinders .177 and 22 pistons are the same .The cost of a barrel alone $$$$ you could get a second hand gun in a poor state and fix it up ,in the caliber of your choise and get a better resu