What you do is on the war machine map you stand in front of the troka (big machinegun) and then switch to your shotgun.
As the guy's hand touches the shotgun double tap X 2 times and then shoot with the shotgun. Nothing should come out.
Now do this with the lance and shoot a few times.
After that ask another person what gun you have out, if they say you have a shotgun out when on your screen it's a lancer... you did it right!
Now go up to an enemy and chainsaw them, to them it looks like you are hitting them with the shotgun but you both can see the chainsaw going. No blood will come out and he will survive.
Now if you fire, hit B, get shot or die and that person will explode no matter what (the person you chainsawed).
This will last the rest of the round and you can also do it in campaign mode but it will end when you get a cutscene.