I went on Socom 3 for the first time since last summer. I played fault small and went into the dough nut room. I jumped up on the cot next to the door and moved towards the locker so I could get as far back from the door as possible. When I did this, it popped up next to my cross hairs: "No Shot".
It means you had something directly in front of the muzzle of the gun, and if you fired, the bullets would hit the obstruction. You may have thought you had a clear shot, but none of your bullets would have made it through.
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"History will be kind to me for I intend to write it." -Winston Churchill