It doesn't take a rocket scientist to figure out the steyr.
You shoot for one second, let loose for a millisecond, repeat. This way you will be spitting 3-4 bullets pretty accurately and very, very fast.
Shoot. All the time. Don't see the enemy? No problem, your bullets travel through the screen anyway. Keep that sweet ratatat going, by the time your enemy gets near you, he should be bleeding in galloons, and it's going to be a sintch finishing him off. In the meanwhile, use your grenades effectively: injure your enemy first, throw a grenade in his face, shoot him again. An injured player will most likely die from a grenade hit, but if your bullet and the nade hit at the same time, it's bound to be an insta-kill too.
With this method, you can mow through most normal opposition like nothing. This also applies to the other autos, it's just as easy with mp5, a bit tricky with minimi, and the Ak requires more precision.