If you can find one take a look at the Remington M-7615. It is a pump action .223 that takes AR15 mags. Mine came with a couple 10 mags but it will take regular AR15 mags of all capacities. They made them for the police market and they didn't sell well. Not sure what the going price is at present but there are some out there.

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Keep Shootin' and check 6