The AR-15 does not lend itself to stock-less use, because its basic design places the recoil spring in the length of the stock, and requires that space for the bolt carrier to travel in. Olympic Arms, however, developed a way to modify the basic AR-15 design to allow for a pistol version that did not require a rear buffer tube (it was also made into a carbine with a folding stock). This was the OA-93, and it was unluckily introduced just before the implementation of the 1994 assault weapons ban in the US.
With that law in place, the OA-93 met the legal definition of "assault pistol", and Olympic wanted to find a way to avoid that designation, so they could continue to sell the gun. Their first solution was to replace its ability to use normal AR magazines with a permanently mounted 10-round magazine - but this was a commercial flop. Looking for a better solution, they developed the OA-98. This version of the gun omitted the barrel shroud and thread muzzle, thus removing two of the "assault pistol" elements. A third and final element was avoided by reducing the weigh of the gun from 71 to 48 ounces (from 2kg to 1.36kg). This was done through drastic skeletonization of several components, and removal of all excess features - and the result was a pistol which indeed avoided the legal definition of "assault pistol."
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