Employees from the muzzleloading maker Knight Rifles tried shooting the enormous .950 JDJ loaded with 2,400 grain bullets! Firing that thing looks like as much fun as being hit in the shoulder with a baseball bat
The rifle uses a shell-holder style action. To load a shell-holder action first the bolt is removed, then the cartridge is inserted into the bolt (which grips the cartridge) and finally the bolt and cartridge are inserted into the action. This type of action is used because it is strong and relatively cheap manufacture.
Note in the video that one of the shooters is told to be very careful when inserting the bolt/cartridge and cocking the action. If anything went wrong, the heavy recoiling cannon could easily crack a skull or break a few ribs!
[ Many thanks to jdun1911 for emailing me the link. ]