Don't use shells that have lil ridges on the outsidee of 'em. They seem to hang up in Semi's more than a smoth cased shell.

Other than that keep the action about 90% free of oil like other ssuggested.

Not sure how the Mossbergs work, whether they are gas or inertia. I am assuming gas. If my tips above don't help spend more money an get something like a Browning, Beretta, Benelii.