I purchased (but haven't' received it yet) a Adams Arms AR-15 5.56x45mm NATO 14.5" Pinned Barrel 1 in 7 Mid Length Gas Piston Complete Upper Assembly.  I want to order a rail system and replace the hand guard (of course) but don't know what sizes work...  Any help is greatly appreciated.

Welcome to the site! You have lots of research to do!

Typically speaking, rails for associated gas system lengths (if the gas block is left exposed):

Carbine = 7"
Mid = 9"
Rifle = 12"

A rail usually needs special consideration to ensure it will work with a piston system. Conveniently, Adams Arms provided you a list of rails that work with their system in their FAQ list on their website.

That said, you've complicated things a bit with your pinned muzzle device. I have no experience with the Adams system, but I'm willing to bet it won't slide over your permanently fixed flash hider. That means whatever rail you buy will have to be a design that uses the standard barrel nut (assuming they use one on the upper in question), and that is also split in half to facilitate being installed in-place.

I'd say get your upper, don't worry about the rail straight away, shoot it lots, hang out here and do your research, watch the EE, and the rest will fall into place.

Samson evolution ex 9 is nice when you have railed gas block or a Troy trx in 9" there are others that fit on the mil spec barrel nut listed on Adams arms website on the FAQ section.  If you have the adams arms xlp (low profile) gas block you have a few more options and can go with a longer rail that will go over and extend to what ever length you like.  Just make sure what ever rail you buy uses either a mil spec barrel nut or is intended for a gas piston gun like the midwestwest industries free float 2 piece designed to fit most gas piston guns (definitely does fit Adams arms).  

If your on a smaller budget and don't care for a free float rail buy the magpul moe hand guard just make sure it's the newer version that clears the piston.

I'm confident with some time and the search function on this site you will find all you ever need to know plus a lot you don't lol. Good luck & btw I love my 16" Adams arms mid length upper is probably the main reason I responded to your thread.  What ever you do if you feel overwhelmed take a second and do more research or ask here.  There are so many options and it is overwhelming sometimes

Listen to the guy above me he's definitely got the right idea research and shoot and shoot it some more

When making that decision I look at several factors:

Gas Block:
*Do I want or need the gas block to be exposed (EI: A2 Front Sight Post)
*3/4 Coverage for easy access to the front of the gas block such as an adjustable gas block particularly on a lightweight bolt/buffer set up where I might be changing gas setting more often for the ammo I am shooting
*Completely covered Gas Block for either personal aesthetic reasons or I am planning on running a flip up front sight (Sight Radius) or weapons light as the closer to the muzzle it is the less shadow is cast by the barrel.

Given your barrel and mid gas length:

9" Handguard will not cover the gas block and allow for a FSB Gas Block in most cases
10" handguard will cover 1/2-3/4 of a low profile gas block and allow easy access if you have an adjustable gas block.  This is also long enough to mount a Weapons Light with some barrel shadow but not enough to have negative effect on function.
12-13" Rail will cover the entire gas block a few inches inside.  This also gives maximum sight radius if your are using irons and will easily allow you to mount a weapons light far enough forward to cast minimal barrel shadow.  This however will also be the heaviest.

I would not put any longer than a 13" handguard on a 14.5" barrel unless I was using a suppressor and wanted the suppressor "Tucked" inside my hand guard.

Here is a picture of my last project I have handy in my photo bucket.  Its a 16" barrel with a 13" KMR.  Seeing as you have a 14.5" barrel the Handguard/Exposed Barrel ratio will be about the same with a 10" handguard if you care about that sort of thing.

P.S. You cannot use a BCM KMR with an AA Kit without modification.


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I also found this thread in another section of AR15.com of a 13" on a 14.5 barrel for you.  This way you cansee the aesthetics and determine what you like.

http://www.ar15.com/forums/t_3_4/689122_My_first_complete_build_and_first_SBR___pics_.html

http://www.adamsarms.net/faq?SID=bbi8hnp0vjtqh390j9bppliuj4#qa1