For an AR I would HIGHLY recomend the KDLitker c8.2 with XHP35 emitter. Its around 2,000 lumens and is VERY well built. I have never had a KD product fail.
http://www.kaidomain.com/p/S027414.KDLITKER-C8_2-Cree-XHP35-HD-Neutral-White-5000K-2200-Lumens-5-Mode-LED-Flashlight-Black
a shotgun is significantly more recoil. I would recomend a thrunite or nitecore for it.