You will probably have much better results if you don't use over shot cards. Comparing shotshells with overshot cards to those without is like comparing broccoli to peaches.

There are some very good 16ga 15/16oz loads in 67mm cases with fiber wads available if you decide to use factory ammo. You can find them by googling "16ga royal game". Of course that's not going to help you with the choke.

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