Here's a short tutorial on how to do the glitch where your gun model disappears in Battlefield 3. I had an accidental instance of this, and as I tried to replicate the glitch, I noticed that using the G3 + M26 combo is pretty much the most certain way of getting the glitch. I tried other assault rifles with the M26 attached, but couldn't get the same results.
Hope you find this useful, at least it's good for shooting cinematics when combined with the No HUD console command.
The video also includes the original instance where I accidentally make my gun disappear, after which I get a funny killing spree with the invisible gun glitch on.