There's no artificial balance on weapon damage.

Its still insanely overpowered, if you can destroy a tank with an autocannon faster than with a normal tank gun, its broken.

I only assigned it as a balance because I thought the value is off.

There are many factors at play. Tank Armor is heavily WIP.

Location on the Armor also matters, along with the caliber of the weapon.

not to mention, AP(Armored Piercing) rounds are meant to stop tanks

This has a 40mm CTWS cannon. It is extremely powerful compared to smaller cannons, such as those on the Bradley or the BMP-3. It fires a tungsten APFSDS round that is said to penetrate 150mm of armor at 1500m.

Anyways, this cannon produces 320 kJ muzzle energy. Based on energy would produce a damage value of about 113. However, it has a damage value of 148, which appears too high to me as well (differences are squared, so 148 is 70% more damage than 113).

What I think MAY have happened? - perhaps someone based energy calculation on the mass of the entire projectile (sabot + penetrator, etc), not the mass of the penetrator alone.

When I did the calculations for the entire projectile, including sabot, I ended up with a damage value of hit = 150, which is almost the exact same as the 148 value currently in the game.

So it seems it should be toned down to around 113 or somewhere in that range. Although, there is the possibility damage is increased for sabot rounds to try to make them more effective compared to a normal bullet of the same energy.

Note, in ArmA 2 this same cannon was on the BAF Warrior, it did hit = 300... way too high.

Also, I noticed the Marshall only holds 40 APFSDS and 60 HE rounds. Is that an attempt at balancing? Because the 40mm CTWS ammunition was designed specifically for the purpose of saving space!

By the way, I tested it on Varuk just now - I shot all 40 rounds of AP, and it did not destroy the tank. In fact the only major damage was to the right track (this was firing at the front armor of the tank).

I have tried again, it will survive 40 rounds from front and back, but exactly 7 rounds to the side will cause an explosion.

That is probably less of a problem with the 40mm cannon and more of a problem with the tank's armor.

Goose: The amount of rounds stored in the Arma 3 AMV is the same as in the real life counterpart, for that type of gun and turret. I think the problem is that the GPR rounds aren't lethal enough: they apparently designed the gun to be a "sniper" rather than "volumne of fire" type of weapons system, with programmable rounds. The real GPR can be set to explode after penetration, to air-burst and ground burst. We don'T have that capability in A3.

Edit: I should note that the gun mounted in that turret by design is not the CTWS, but the Denel land systems GI-30 cannon (this is also the one modelled visually, I think), ammo count is based on that. (The systems combination is the LCT-30 turret with the GI-30 gun: http://www.army-guide.com/eng/product3096.html )