I'll try to explain, but not to make this too complex and i'm doing it from memory so my figures are all approximates. It's not as simple as what an Apache can/can't withstand
The Apache is a very robust Helicopter which can take a lot of abuse/hits. However a PKM fires a 7.62mm x 54mm round. The round itself (i.e the bit that comes out the barrel) is 7.62mm diameter and 24mm long. The PKM has a cyclic rate of 600-850 rds per minute depending on the variant. Cyclic rate basically means that if you had a never ending belt of ammunition and kept your finger on the trigger it would fire upto 850rds in one minute or about 14rds a second.
Now the insurgents bullet of choice for these is the Armour Piercing, or AP, bullet which is a Heavy Steel/Lead core (depending on manufacturer) sheathed with a copper jacket. At 1000m this round is still packing over 2000lb/ft of energy and weighs about 25grams (vs 20 for the non armour piercing). At 2000m it is still carrying over 1400lb/ft of energy. Although the effective range is supposedly 1800m i wouldn't want to be stood any closer than 3000m without protection.
Now although the Apache will take a lot of abuse and hits the insurgents don't control their rate of fire, they spray and slay as we call it. 14 AP rds a second can do a lot of damage to an Apache. The Americans have had one shot down due to AK47 fire (which is a far shorter/far less powerful round in terms of penetration fire) but they were lucky hits that knackered the disc (rotors). The Cockpit will only protect really from ricochets and non AP rounds and then you have the Hellfire's and rocket pods on the outside. If you imagine a Hellfire missile is something that is angry but stable, it then gets hit with a couple of AP rounds and it suddenly becomes very angry and very unstable and it's something the pilot doesn't want sat on the rail. He can't fire it off incase it causes premature detonation taking the AH down. I have seen how the Bomb disposal guys have to deal with one of those and trust me i'd much rather face whatever the insurgents can throw at me.