This is two-foot tall replica of the engineer's sentry gun from Team Fortress 2, created from hundreds of LEGO bricks.
ZaziNombies did a good job on this one, and the mounting doesn't even look like LEGO. Now do the level 3 version of this with the twin-mounted gatling guns. That'd look badass.
LEGO Sentry Gun - Team Fortress 2 [YouTube]
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