Dave,
Please do not shoot the 1500 fps loads Marty has sent you in your fine Simpson. That load is for the Westley Richards Fauneta ONLY! By the looks of your target, your Simpson is, as others are suggesting, regulated for the lighter Kynoch type load - roughly a 425 gr bullet in the 1,100fps range.
In my experience with my (12) paradox type, the low crossers you are getting are indicative of a velocity that is too high. I know the slug Marty uses and, when I cast it, it ways about 425-430. The mold and design were made by John Millar specifically to shoot from a Fauneta. You should get the slug to work out of the simpson, but at lower velocity.
Not sure how heavy your bbls are, but the Fauneta has really heavy bbls. The thickness at the muzzles is maybe 3/16" (guessing, but "really thick for a shotgun").
You do not need the Primex 513 that Marty loads to regulate in the WR Fauneta's. Im guessing here, but I'd start with something like Unique or Blue Dot with a light load and work up carefully with the aid of a chronograph. If you are normal and lack time, you can send your Simpson to someone like Tom Armbrust who will work up a load for you. I'd be happy to help with suitable slugs for the Simpson.
Hope you find this helpful. Reading your post made me really scared for you and your Simpson.
