Is your ultra one that cocks via the mmc action in front of the stock or bolt action at the rear?
I have a hornet and an older scorpion that have the older magazines that are indexed via a pin linked to the trigger mech , I think if yours is the same then the new magazine may be blocked by the pin in from slotting into the mag housing. I do have a few modern magazines that have a cut away so they sit over the pin.