I have shot .50, .60 cal machine guns from my ship. Shot AR-15 9mm and .308 and a Romanian AK-47 at an outdoor range that friends own.
My AR-15 .223, 55 grain (pics in Reba's post) is the best for firing all day long. Max range 600 yards. The bullet tumbles at distance and rips into you. I can also use 62 grain hollow points. The military M-4 that is currently in use is 5.56. The ammo is not interchangeable. The 7.62 is also not compatible with the .308.
The AK-47 sounds like a rattly JD tractor at speed. It is great for spraying targets. If you want accuracy, use an AR-15. It only sounds like a large spring snapping next to your ear.
If you want to knock down walls, attach a M-203 40mm grenade launcher under it. For civilian use you can use the 37mm and put in flares or smoke grenades. Great Christmas gift for your loved ones.