First off, you can't be host to do this glitch.

1) Take you Lancer and shoot a few rounds from it, but not the whole clip.
2) Get an ative reload. But when you would press the reload button to get the reload, you also have to press the fire button and melee button. So, in other words, when the bar goes into the active reload area, press RT+RB+B all at the same time.
3) Now, keep those buttons down. If done correctly, you should be firing blanks right now.
4) Quickly switch to your shotgun and as soon as you have it out, let go of all the buttons.
5) Shoot the shotgun once and you're ready to go.
6) Press and hold A and run into someone. There should be a chainsaw animation, but no blood and it won't kill the victim. You have your hostage now.
7) To kill him, switch to your Lancer and he will blow up. Also, if you get killed, it will also kill the hostage.

DON'T DO THIS GLITCH (OR ANY GLITCH FOR THAT MATTER) IN RANKED MATCHES!!! It's best to do with your friends who won't boot you or file complaints.