The Fourpounder gun from Deathloop ripped directly from the games files.
fourpounder.stl - The file directly from the game converted to stl
repaired fourpounder.stl - run through 3d builder to fix non-manifold surfaces
the other two files - the repaired version split up and with the barrel fixed and extended all the way back to the mag well to make slightly more sense

How: http://aluigi.altervista.org/quickbms.htm
https://forum.xentax.com/viewtopic.php?f=16&t=24503&p=178125

For anyone wanting to repeat these steps to get their own favourite items most of the models for equipment and world objects are in \generated\model\models