Well that looks very bad.

And they also need more spread.
/upvoted

I've been playing a lot as a CAS pilot on the pawnee and when you run out of 'Hydras' and have to switch to the minigun it took me around one minute to kill a simple infantry unit.
Two reasons for that , the very low damage of the minigun and the fact the helicopters controls in ArmA3 are awefull, since the helicopter move too easily the crosshair just dance around the target making it untouchable while in ArmA2 helicopters controls were smoother making easier to aim correctly.

Added real life video in Additional Info.

Did you have extended armor on? Is on by default. Go to CONFIGURE -> GAME -> DIFFIULTY -> Extended Armor -> Disabled.

It's not extended armour. They do the same amount of damage as a 6.5 round. The RoF for the miniguns is extremely low and this combined with the pinpoint accuracy (#0001834, #0001747) they have makes them pretty useless.
The RoF is tied to the framerate (#0009828) so unless they add more shots per frame or add some splash damage they are always going to be terrible.

I did not ProGamer. However, should civilians even have "extended armor"?

Every unit I think has it when it's enabled though I usually play with it off so I am not exactly sure. Try testing without extended armor enabled though. Frame rate and rate of fire are definelty issues for these weapons.

All 5.56 and 6.5mm in game have low powerful. Kill somebody after penetreiting wall - mirrical!

http://youtu.be/gK7sMGJJftk

They fixed it. They appear to have added a small amount of splash damage. It works as you would expect now.

It still needs some tweaking, the splash is a bit too big and quickly demolishes buildings. While its not entirely unrealistic for bullets to break a building down its a bit too fast. In fact its easier to destroy a building with the minigun than it is with the rockets.

I dont know if its possible but it would prefered to do different damage to building classes than it does to people classes.

Sadly adding spash damage doesn't fix the underlying issue the miniguns have and creates the problem that you reported of being unrealistically powerful against buildings.
BI has to make them less accurate and have a realistic rate of fire: from what I could test, the miniguns on the AH-9 have a _combined_ rate of fire of around 1400rpm and with good FPS (50ish).
That means that each minigun has a ROF of around 700rpm, which is nowhere near the minimum of 2000 that those thing can reach, and even farther than the maximum rate of 6000rpm, and that's when the game doesn't slow down, reducing the ROF we already have.

What about going back to the Arma 2 fix: http://feedback.arma3.com/view.php?id=15519 until Arma 3 can have the engine limitation fixed?