My Gates it seemsthat we are working on a similar project, to create a modern 12 Bore gun/ load combination. I have tried several slug designs already, and realized that the twist rates in the modern barrels are way to fast. Most of the loads that you and I are working on need aslower twist for best accuracy and penetration.I recently had a custom 1in84" twist barrel billet made, it is not yet fitted to an action. It should be the appropriate twist accoding to the Greenhill formula. I have a custom 1in28" barrel on my threaded Ithaca 37 for shooting sabots. When I fire my 615-765 grain slugs from the rifled barrel, the slugs yaw in the target. My smoothbore fires then perfectly square into the target, and shoots 1-2 inch groups at 50yards. The design of my slugs are hollow base, semi-wadcutter style. The 615 grain slug is moving about 1500+ FPS I am using a single base powder,as opposed to the Blue dot. Blue dot can give decreased performance in cold weather. I look forward to chatting with you on this project. Greg Sappington