I'm not a full auto expert. I just shoot a lot of it, and I'm more knowledgable than the average person on this issue, but I am far from being an expert.
Here is how I do it.
1. Use an athletic stance, like a football linebacker stance. Position your feet a little more than shoulder width apart. Your weak side foot should be about 1-2 feet on front of your other foot.
Do not use a semi auto shooting stance, like a Weaver shooting stance. By using a Weaver stance, you allow your upper body to be used as a fulcrum by the recoil, thereby causing your upper body to be pushed back by the guns' recoil. The Weaver stance feels comforting because you think that your rear foot is going to push back and brace you against the muzzle rise. This is a false comfort, as the muzzle rise will come as a result of your back being forced, by the recoil, to straighten up; and your feet won't help you with that.
Another important reason to use this foot stance is that you will be able to effectively move and shoot. I was taught by my instructor to shoot and move, and not be a stationary target, which is an important component of CBQ. By spacing and taking the foot position I described above, you can move and walk by doing the 'tactical shuffle', which is a stable moving and shooting position. Also, this position will allow you to seamlessly transition and move on your knees, while ducking, which my instructor called the 'tactical duck walk'.
2. Shift slightly more weight onto your front foot. The weak side leg, that is in front, should have 60% of the weight.
3. Bend your knees in an athletic manner. Do not lock your knees.
4. Lean forward slightly, meaning to cant your spine forward at a 15-20 degree angle. This step is very important. Standing straight up with cause you to quickly lose control of the gun, once full auto starts. Do not have a straight up stance, as this is the primary cause why the second shot will go above the target.
5. Shoulder the gun. The gun should be shouldered on the same half of your body that has your rear foot. When shouldering the gun, bring it in deep into your shoulder/chest joint. An alternative way to is shoulder the gun center chest and high up on your chest. The advantage to this is that you don't have to duck your head into the sight, as looking down at the ground is a deadly sin in CBQ (the ground isn't going to shoot you and kill you, so you should never be ducking your head and looking at the ground; rather look forward to where your enemy might be...wise words from my instructor).
If you take the above positioning, you can mag dump with ease. Concurrently, you can then learn trigger control, but trigger control isn't a substitute for proper technique. If you just control the gun by using trigger control, you might be hiding bad shooting technique.
Here is a YouTube video of a guy who is totally shooting the gun incorrectly, and then blaming the gun.
https://youtu.be/NvQSN4aBGPs
Compared to this guy who runs off the entire belt...
https://youtu.be/5aepTyJ_fsg
Both guys are shooting M240s.
MP5s are easier, as the recoil is minimal.