So im putting together a new ar with the upper a carbine length gas system, the thing is i want a free float hand guard with a low pro gas system so i can put on rear and front flip up sights, my worry is the gas block will be exposed by the hand guard(carbine length), is this true? should i buy a mid length hand guard thats just a couple inches longer or so? thanks guys

Most people that run lo-pro gas blocks always buy rails longer than the gas system they are running so the gas blocks are covered.

I would go at least mid-length with a 16" barrel. If I were building a rifle like that I would probably use either a 13" troy alpha or 12" Midwest Industries Gen 2 SS rail

I'm assuming a 16" barrel? If so then you can use any length hand guard out to 15"  Anything that is 8" or longer should cover the gas block on a carbine gas system.  Get a low pro gas block and whatever length rail you like best and go to town.  I'd be cautious about weight, some FF rails are heavy and that's the worst place to but extra weight on a gun.  I love the Fortis Rev, it's 12" and <10 oz, feels perfect to me.


edit: Fortis Rev

Go with anything longer than a Carbine rail (7"), since you'll be adding it anyway might as well step up to around 12" or so (10" is still fine if weight is a concern) to extend your sight radius.  You can go with a quad rail (gonna be a little heavier) or go with something Keymod.  

Depends really on what you're looking to mount and what you're looking to do with it.