Double feed on mossberg 500AB-12 guage - Mossberg 500 Pump 12Ga 3" 20" Parkerized Pistol Grip w/Heat Shield Cyl

Push all the shells into the tube ALL THE WAY with the first half of your thumb. Try shooting it again. The most common "double feed/one drops onto the ground" is one shell not all the way forward of both stops in the mag tube. If it's only one stop, it drops to the ground. Second, if that didn't fix it, try stripping the gun and cleaning the slots the two stops rest in. Both have to be spotless without grit or the stops won't work right. Third, if both the others didn't fix it, email Mossberg and get replacement stops....someone "fitzing" the gun without knowing what the heck they were doing is the third most common fault in feeding. You can get them from Ebay cheaply but chances are they may be the bad ones someone else replaced.