IMO there is not much that can be done to improve a 500/835 trigger unfortunatly. There is actually a weight pull adjustment screw in the trigger group but it wont do much to help the creep. I couldnt find an aftermarket trigger ,so I took one I owned once, to a GS and he said anything he could do wouldnt help accuracy much and not to waste my $$$. He suggested "pinning" the barrel (to the reciever)if anything.

My mossberg threw bren, ko sabots into a 2.5" group at 100. I dont think you can expect much more from a pump. Kicked like hell though, so I got rid of it.

Look into a savage 210 bolt action slug gun. Im not sure if they come w/ accutrigger but older savage, bolt action triggers, were pretty good and easily adjusted.

http://www.savagearms.com/210f.htm heres the link. No accutrigger unfortunately. Ive seen them at gander mt for around 450$