A Mossberg 395k has an adjusable choke on the end of the barel. Start by opening your choke to one turn past the improved cylender setting when useing slugs. This will eliminate the problem of too much choke, which can cause accuracy issues with shotguns. Secondly, is the miss a left\right or up\down? Up\down has only one fix. If you are hitting 2 inches low, aim 2 inches high. This is known as Kentucky windage, and it has worked for hundreds of years. Left\right you should be able to adjust by drifting your rear sight, which on that model is located in a dovetail groove just after the receiver on the barrel. Move it left to move the POI left.an right to move the POI right. When using this shot gun with shot,the rear sight can be removed and you just aim with the bead and the top of the reciever. I own two similar mossies, a 185d and a 185k-a. The d has a rear peep sight that is adjusted in a similar manner (it does do up\down) and that is how I sighted mine in. It will take out three beer cans with three slugs at 50 yards, so yours has that kind of potential. If you are dealing with a bent barrel, ask a local gunsmith for help.
I hope this helps, Post with an update or PM me to let me know. Good luck!
P.S. Check to see if the assembly screw on the bottom of the stock is loose. This can cause the barrel and action to move on tis own when fireing and send your shots all over the place.
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