Summary

The Fourpounder gun directly extracted from the Deathloop game files.
fourpounder.stl - A file converted directly from the game to STL format
Repaired fourpounder.stl - repaired the non-continuous surface with 3D Builder
The other two files - fixed the version split and repaired the gun barrel, extending it to the magazine hole, making it a bit more reasonable

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

For those who want to repeat these steps to get the items they like, most of the models of equipment and world objects are in the \generated\model\models directory.