Keep same number of rounds after switching weapons:

Start a multi-player game and get the heavy 
weapons character. Have your Gatlin Gun ready and
get ready to switch your gun. Have the gun that 
you want, ready then hold the Left Mouse Button 
for three seconds. You will hear the gun rotate. 
The gun you selected will have the same ammunition
that the other gun. This also gives you an extra 
192 shots with the shotguns.