IF YOU DOWNLOADED THIS BEFORE JUNE 13, RE-DOWNLOAD IT FOR AUTO-UPDATES. Enables viewmodels to shift with certain events. Included is the data to flip the CAR-4 over on empty reload when hitting the bolt release. Other mods may use this as well. Don't download this if you're already running IREnFIST. This is ripped straight from it. Extract to PAYDAY 2/mods

Also supports the following custom weapons right out of the box (not coincidentally, the custom weapons using the CAR-4 animation supported by InF as of the initial standalone release):
AAC Honey Badger
HK416 (by >:3)
HK416 Standalone (by carl)
Remington ACR

Modders, check wpn_stats.lua to see how easy it is to use the underlying system for fun and profit.