I know this is an older post but on the Remington 510-511- single shot you have to countersink the barrel as the 22 mag has a thicker head. Do it a little at a time until the bolt just closes and with the barrel removed make it a little deeper to the rifling. It is helpful to have a 22 mag rifle to measure. I know where there a barrel for a 510 if someone needs one. The barrel is all you need for a mag and you can switch the barrel back. Easy swap.