Firs things first, I searched and didn't find this anywhere on this forum. I'm also sure this has been found somewhere else because I'm not the best when it comes to finding these things, so if I found it I'm sure others have too.
As an MM hunter, when you have 4 stacks of MMM (Master Marksman) you can cast your last Steady Shot and prime an Aimed Shot on the end of it in order to get 2 free Aimed Shots instead of just 1.
How it works:
After your steady shot is fired, if you primed Aimed Shot, you will be mid cast when you get your 5th MMM stack and it will proc your Free Instant cast Aimed Shot. Since you are already casting Aimed shot it doesn't eat your "Fire!" charge but it becomes free as if it does. So now your casted Aimed Shot is free and you can still use your instant free Aimed Shot.
Doing this you can get about 120k (assuming they both crit) damage in about 2.5 seconds for no focus cost. Add in an instant Chimera shot and you can get up to 150k (these numbers are based on my gear)