Let us know how the mossberg 100 ATR shoots (I have been looking at one for a truck gun).
As for a good under 400 scope I would look at a 3200 Bushnell, or a Nikon Monarch UCC. These are excellent scopes and can be had for well under your price of 400 or less for a scope. Check out www.SWFA.com
they have excellent prices and awsome service. Also, might want to check out the supersniper line of scopes there as well. SWFA bought the rights from TASCO and are now making them to the original specs again and are very good.