this is for MGs with sights that vary between 1.2 and 1.4
grab a nearby object something like a paper weight. Place it on your mouse pad. Grab an smg or AR and enter shooting range. Now aim down sights at one of the target's head. Then place mouse up against it. Now rotate 90 degrees until you crosshair is over the other targets head. Place another paper weight up against mouse where your mouse ended.
Increase the sensitivity of your mouse until you can rotate over both targets the same way while covering the same distance on your mouse pad.
Only thing you have to worry about is when trying to knife or hipfiring, your mouse is really sensitive.
I use the SSE5 grip attachment for ads accuracy since that's where I spend all my time.
I find it easier to handle hipfire with higher sensitivity than to have to handle really low sensitivity while in ADS.
You could also try something in between if you want.
proceed to wreck havoc
Good chipsets for machine gun: Agile step is a must, maybe rapid aim, maybe handgun stability or rapid swap.
Machine guns take time to draw when weapon switching so if you don't have time use handgun. say you're running with knife out and see an enemy, go for handgun.
the other option is to get a 2.0 or 3.4 sight the sensitivity should work fine. only thing is its for ranged targets.
if you really like to hipfire I would go with that, and get either AFG grips.
then again if you like to hipfire don't get an MG.