First time I heard the of the concept of the “contact mag” it was actually the “contact belt”.
The M240 gunners, carrying an somewhat large and awkward gun, would carry it with a short belt of 5-10 rounds. That way, when foot patrolling and running into unexpected enemy contact, they could fire off a burst or two as they moved to cover or a better firing position. At that point, the assistant gunner would hook up a full belt and they would rock and roll.
Later I saw two different takes on the concept for use with the rifle. Opposite ideas.
Back in the good ol’ days we all had the same mags. Either all 20s, or all 30s. When I was in Iraq, we all carried 30 round magazines. It was so simple.
Now we have everything from 150 round drums to sleds for single loading rounds in competetion shooting.
One group says that you need a light handy rifle capable of engaging an enemy that has briefly exposed them selves as well as you only need a few rounds to provide a little covering/suppressing fire as you move to cover. If the engagement lasts long enough to expend the rounds in your light & handy contact mag, then you switch to your standard mags for the rest of the combat encounter.
Imagine if you are using a 20 round mag in your rifle. If an enemy tries to take a pop shot at you from a 3rd story window, you can quickly engage them with a few rounds from your light handy rifle. If you get ambushed by a superior force, you can dump those 20 rounds as you move out of the kill zone. Then once you are in cover and or concealment, you reload and continue to fight.
The other school of thought is to have as much ammo in the weapon as possible. The highest capacity mags. When you encounter the enemy, you respond with overwhelming fire. The idea is to immediately gain fire superiority. Hopefully, you will route or destroy the enemy with the fire power from that first magazine in your weapon. If you end up depleting this extra high capacity magazine, you dump it to free your self of that weight and you continue on with your normal magazines. You will likely lower your rate of fire during this transition in order to ensure you can continue to fight no matter the duration of the fight. They recognize that carrying a bunch of drums or giant magazines may not be practical, so they advocate having a single drum or very high capacity mag in the weapon, and the rest of your load out being standard magazines.
Which idea is correct?
They both are. Mission, environment, terrain, time, and the situation dictate which you use.
In any event, I need my weapon to be maneuverable. Either I make it light enough so that I can move it fast enough to make what ever shot I may need to. Or I become strong enough to be able to manhandle the weapon the way it needs to be employed.
If I know that I am moving to make intentional contact with an enemy force. I want as many rounds as I can stuff in my weapon. If I am just doing a long presence patrol in a low hostility area with plenty of places I can take cover. I can use a lighter, smaller, magazine to help prevent fatigue and ensure I don’t snag on the environment, can move faster, quieter, etc.
You might transition between the two. If you are assaulting an enemy position, you might have a smaller mag in your weapon to stay lighter and quieter in the movement to the staging area. Then switch to your highest capacity magazine in the staging area to have superior fire capacity for the initial assault.
Still, all of this relies on you have a variety of options and choosing which one fits your situation and tactics. Something to think about.