Using CQC in Metal Gear 3 in the PS2 you could slip a soldier's throat by pressing hard the X button or hold the solder by holding the X button in a moderate pressure. How that will work in the Xbox controller? Forget me if I'm a noob, but I don't know if the Xbox 360 controller has pressure-sensitive buttons, or if it's either activated or deactivated.
I've always been under the impression that the face button on the Xbox360 controller are not pressure sensitive. If you read Destructoid's review of this collection, he states that you press press the analogs since the Xbox360 controller doesn't have pressure sensitive face buttons.
"It should also be noted that the Xbox 360 controller doesn't feature the pressure-sensitive input that were a big part of the combat in these two games. However, aiming and relaxing guns are handled quite adequately with a click of the movement stick, which I actually find more manageable than dealing with varying degress of button pressure. " http://www.destructoid.com/review-metal-gear-solid-hd-collection-215893.phtml