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An Auto-5 with the barrel cut down flush with the magazine tube. As the recoil spring is fitted in an assembly inside the grip of the stock (and the bolt carrier connects to it with a pivoting link), you can't really cut the stocks of these shotguns down very much. I imagine that the drastic reduction in the mass of the recoiling barrel may affect reliability with some loads, but I couldn't say for sure, likely you may want to play with the setup of the friction-rings (potentially you may want a custom set of friction-rings for optimal performance and wear reduction).
Clyde Barrow, fabled counterpart of Elizabeth Bonnie, carried a very short Auto-5 in a shoulder holster kind of rig, calling it his "Whip-It-Gun", as he could rapidly whip out from his coat/jacket and have it shouldered in a second.