Guys I think his question was, which can be used as a hand-held side-arm type deal for a sniper rifle. IE, he wants to have his sniper rifle, and then some AR15 type rifle as a secondary...
You can make a mad stubby version. If you want to go for un-realism, I've seen people get little sling-attachment end caps for their AEGs. It will be unrealistic, since no matter what an AR15 has to have a buffer tube IRL, but it will be small. And then a super short front end kit. Not much stock comes like this. The only thing I can think of is the WA Patriot GBBR pistol. You would need to make one yourself from scratch or by buying an M4 and then replacing the stock and front kit. In which case, it doesn't matter what you start with, just whatever is available to you.
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