I had the same problem with my Mossberg 930 Black water. and I found the problem, I Bought 150 rounds of Federal Premium Law enforcement Buckshot and 9 of the 150 rounds had to be forced in to the barrel with my finger because these rounds had bulging buckshot showing on the outside of the casing ( bumps ) that would not allow the bolt to close all the way allowing to much clearance between the firing pin and the shell. This was found when I manually fed each of the rest of the shells in by hand and found some of the shells had to be forced in the last half of the way. I went to the range the next day and fired 50 of the shells I checked were good and all fired like they should have. I also had a Beretta 12 auto that wouldn't fire some of the cheaper no# 6 shot for the same reason (bumps) on the outside of the casing. Hopes this helps, Now I will buy one box and dump it out on the table at home and check each one and I If I find some bad ones I won't buy that brand again.
John, Covington Ga
