Pretty simple, just spam the ever living christ out of R to cycle your weapon and it will keep going as long as you keep mashing. I highly recommend making a macro for it unless you want carpal-tunnel.
Edit: The following weapons do NOT work with this glitch.
* Izh-18 Rifle (AKA the Musket)
* Double Barreled Shotgun
* Revolver
* Colt Derringer