Here I will outline the most useful grip methods for Airsoft battles when using an AEG, GBBR, or similar rifle. From here, I will refer to the support hand only (the hand which holds the handguard).
Thumb break grip
The thumb break grip places your four fingers along the bottom of the handguard, with the thumb placed along the side, facing the target. This method can also be used with a vertical foregrip, in which case the fingers go around the grip but the palm stays open and the thumb sits along the side of the handguard, facing the target. The thumb break grip allows the shooter to use the direction of the thumb to point the rifle instinctively, and apply pressure to steer the AEG or GBBR onto the target.
Magazine Well grip
The mag well grip is similar in principle to the thumb break grip, the difference being that your fingers wrap around the magazine well, with your thumb pressing against the side, facing the target. This method is useful for shooting in tight quarters and unconventional positions, and some users will find it makes cornering easier in CQB. Even if you prefer a C-Clamp, it never hurts to practice a mag-well grip as you may find yourself in a position where you do not have room to extend your support hand out without getting hit.
However, this is not a grip you will generally see a trained professional use or those with military experience as on a real weapon it has its drawbacks on real firearms.
C-Clamp Grip/Thumb over bore grip
The C-Clamp grip is an evolution of the thumb break grip which came about as the vertical foregrip began to wane in popularity. This grip places the four fingers along the bottom of the handguard as normal but sees the thumb placed on the upper surface of the handguard, providing down-force to counteract the muzzle rise a real rifle would encounter when firing quickly. The Angled foregrip, or AFG, is specifically designed for this gripping method. However this grip is not really needed if the gun you are using has no recoil since there is nothing to stabilise and merely just makes a rifler user look good rather than having an actual function.
This grip is often seen on Airsoft battlefields in its grossly exaggerated form, with airsofters gripping the very tip of their handguard with their elbow rigidly locked in what looks like (and is, for most people) an uncomfortable position that resembles a chicken wing. This grip does not necessitate gripping further on the handguard than is comfortable and can use a locked elbow, but not at the cost of comfort or flexibility.
Rifle Techniques - The Good and the Bad
Learning and practising all of the above techniques is very useful, as there may be certain scenarios where using your preferred technique may not work or certain rifle setups where the advantage of certain grip styles (C-clamp on a rifle with a short 5-inch handguard, for example) may not be as clear. Whichever way you choose to grip your rifle, the following is a good guide to what works, and what doesn't:
GOOD
Head up, shoulders down - A relaxed position will help you react quickly to sudden threats.
Bring the sight up to your eye - This helps you keep your head up and situational awareness high.
Thumbs towards the target - Encourages more instinctive pointing of the rifle.
Stock entirely in the shoulder - That's what it is designed for! More comfort, cheek weld, better support, and better sight alignment!
Elbow tucked in, minimizing silhouette - Smaller target = Lower hit probability.
BAD
Elbow sticking out - A fantastic way to get your elbow shot as you come around a corner. Elbows should be bent and pointing downward.
Stock above or next to the shoulder - Unless you are using a long fixed stock, this not only looks silly but it offers no benefit at all. It is not stable, diminishes accuracy and is just uncomfortable. If the eye relief of your optic is that bad, bin it!
Bending the neck to see the sights - Classic new shooter error, this limits the movement of your head and makes it more difficult to react to sudden threats.
Shoulders raised - Raised shoulders often indicate tensed muscles which will limit the speed of your movement when rotating your body.
Nice information on grip style! I much prefer the Gangbanger one-handed side-aim style. Feel so thuggish! Also best snipers know you gotta put your eye right on the scope.