I had two Parker lifter 10ga hammer guns. One had 2 5/8" chambers with a sharp step - no forcing cones, and the other 2 7/8" chambers. With a 10, you'll need to reload or buy RST shells. With the 12, even with a tad shorter chambers, you can still shoot 2 3/4" nitro low pressure shells. Some guys even shoot regular modern shells, but that's up to you.
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Paul Harm
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