every time we pattern a gun in we use some cheap 2.75" shells at 20 yards, 30, and 45 yards. Once you feel your close throw a turkey load in and confirm if your sights are on.
FYI I have seen several Mossbergs shoot high. When we sighted in two guns last weekend the Moss 835 was maxed out on the sight and still shot high....if you dont like your sight setup this may be a solution...
http://huntingnet.com/forum/tm.aspx?m=2644567worked great for me.