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First, come up to any cover, such as a corner of a wall or pillar but don't take cover.
Hold Forward with the Left Stick and tap A twice, except hold down the second A.
(You will not be holding any of these button combinations for longer than 1 sec.)
now, while you are going around the corner, slide your thumb so that you can hold A+B at the same time. Right before you stand up, let go of A+B at the same time and walk backwards.
This is a VERY hard glitch to start out with, it took me an hour to perfect it.
This is a working TUT
If you want Infinite Ammo while Crabwalking, shoot one bullet out of your lancer, then hit reload. While you are reloading, press B and then hold down RT. The enemies won't be able to see your bullets, but they do damage. You can hold down RT as long as you want.

^