both actually... i "short-shuck" is when you eject an empty but do not pull the slide far enough back to drop a new shell out of the magazine and onto the bolt. generally speaking it's a problem when shooting fast, and it's a problem with guns that have longer pulls. it's entirely human error, but it's influenced by the type of gun you shoot. that's one of the reasons i only shoot my 3 1/2" 870 for turkeys. for everything else i find myself faster and more reliable with my 3" bps or my 2 3/4" model 12.