yeah your right there about the nozzle on the front of the mechbox. however if there was anything wrong there when you drop BB's in manually nothing would happen if it wasn't functioning right.
Are you using Marui mags on the gun? I ask this because the release on the bb stop on the mag needs to be perfectly aligned or no BB's will eject from the mag into the chamber. if the catch isnt depressed enough you wont get any bb's feeding! As i say i dont recon its a problem with your mechbox so don't panic just yet lol. however if the peg on the body of the gun has been chipped or is notched you'll need a new body probs as I've never come accross a way to fix that problem :S
"Creator of the Zombat Sniper - oh yeah baby"