Grab and Go: SIG Sauer Adaptive Carbine Platform 

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The utility of handguns is obvious. Compact, durable, reasonably powerful and holding a useful amount of ammunition, handguns have been with us for as long as gunsmiths could craft them. However, there has also always been a desire to make them more useful while not losing the good parts. I don’t believe there is a single military organization through the centuries that hasn’t experimented with handguns-as-compact-carbines. Most failed, some miserably, some with a vestige of dignity.
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Famous examples such as the Luger carbines and Artillery Lugers, C96 Mausers and their buttstock-holsters are truly lust-worthy—and ferociously expensive. Even Samuel Colt took to fabricating shoulder stocks for his revolvers. Alas, such approaches fall short for many of us on two details: Adding a stock to a handgun greatly increases its size, and doing so requires paperwork and a federal tax of $200. (More on that in a bit.)
But there is a way to make a handgun a more stable aiming platform and not incur the transfer tax needed to make your handgun into an SBR (Short-Barreled Rifle): the SIG Sauer Adaptive Carbine Platform.
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The ACP is a housing into which you install your handgun. Obviously, it accepts SIG handguns, and the Mk25 SIG, in 9mm, was the test gun I used to try out the ACP. However, the SIG engineers were far more clever than that, as making such a housing such that it only accepted SIG handguns would have been short-sighted. No, they made it adaptable, and as a result it will accept any of a long list of handguns.