G'Day Fella's,

NitroX, as Claydog mentions, it appears to be a reflection off the radiused surface/s (on both sides), just forward of the receiver ring.
Have a look at the image of the floor plate, and you can see same these radii.
Also, the same gap/reflection you mention but on the RHS, has an interruption in it (toward the bottom), which appears to be the Shotgun barrel's latch etc.

Regards
Homer