The Skif insertion tool is adjustable so that you can push different pellets to the correct depth, the pellets need to be below flush at the skirt, firmly seated and leave enough clearance at the front for the mag feed & lock pawls. (take the slide off & the mag out & look down the mag slot while operating the trigger)

My Skif works without jams with Gecos, Accupells, Promethius (metal tipped plastic type), Zaps and Umarex. Each just needs inserting to the correct depth
Nick