When you equip the XMG you get two of them at the same time, you cannot use a XMG on its own. It can be locked down by pressing left on the D-Pad. It is highly inaccurate when fire without being locked down.

The XMGs are fun to use and stylish but generally aren’t very good. You have to be in lock down mode to have a reasonable chance of winning a gun fight, but it takes half a second or so so you can’t activate it when you see an enemy. Hence you have to find a spot and sit there in lock down mode and wait for enemies to appear. When they do they often have a better weapon than you do and can move around to make it harder to hit them. However, you will appreciate how cool you looked in the killcam.

The Best Heavy Weapon
Best Primary Weapon in Advanced Warfare
Pytaek Best Class Setup
All Heavy Weapon Guides

XMG Best Attachments Call of Duty Advanced Warfare

The best attachments for the XMG are those that increase its fire power and accuracy. Use the Laser Sight to reduce the hip fire spread letting you hit enemies more reliably. I would recommend Rapid Fire, which  increases the fire rate of the XMGs making it easier to kill enemies, but it is very difficult to get 3 double kills with the XMGs.

XMG Best Perks of Duty Advanced Warfare

Use perks that protect you and let you find enemies more easily. Low Profile keeps you off enemy UAVs. Flak Jacket reduces the damage you take from explosives. Peripherals increases the distance from which you can see enemies on the mini map. Blind Eye makes you invisible to enemy air support (other than UAVs). Toughness reduces flinching when you are shot at. Due to the high fire rate and low number of bullets available to the XMGs use Scavenger to pick up ammunition from dead enemies and start with more.

XMG Best Loadout Call of Duty Advanced Warfare

Here is the complete loadout. There are 2 points left for scorestreaks.

XMG Best Class Setup Call of Duty Advanced Warfare