Hi all. Got a first timer here. First time building and first ar owned. I have PSA lower to start from. Have many other parts (LPK, upper, BCG, furniture, etc) ordered from PSA, sounds like everyone else does too. Anyway this question is regarding gas system length. I want to go with a 18in barrel. Looking at one from either Noveske or Rainier. I see rifle and intermediate and mid. Can someone please explaine benefits or cons of one to the other. I understand that they are different length and such just need a functional explanation. Thanks in advance. Have already learned so much here, truly a great resource!

Rifle length is designed for 20" barrel
On an 18", the gas port can be opened up a tiny bit or it will be slightly under-gassed.  Shouldn't have any issues.

Intermediate length is designed for 18" barrel
Shouldn't have any issues.  This gas length isn't as standardized/popular, not that there is much wrong with that.

Mid length is designed for 16" barrel
On a 18", it will be slightly over-gassed.   Shouldn't have any issues.


Personally, I would go with rifle length or intermediate length (and that's what I did - 18" rifle length from BCM)

Gas system length is covered pretty well in the previous post...I'd trust both Noveske and Rainier to have them gassed properly, regardless of which length system they use.

I will say this.  I have a Noveske 18" SPR barrel and it is freaking accurate and shoots great.  I've had 20" bull barrels that couldn't touch it.

Rainier Arms is also awesome (I have one of their rifles too).  Although I have no experience with Rainier's stainless barrels, I've heard they're great and have considered going with them for a future 6.8 build.  

You're looking at the two I would be looking at for a stainless barrel...that's gonna be a tough decision for you.

Thanks for the info. Last question is what length hand guard/rail  would cover a intermediate gas block? Want it to be under the rail.

Here are a couple of threads for reference:
http://www.ar15.com/forums/t_3_66/266108_How_it_Works__The_AR_gas_operation_and_how_everything_works_in_harmony_.html
http://www.ar15.com/forums/t_2_130/165511_How_the_Piston_in_your_DI_AR_works.html
The gas system length to barrel length factor changes the function of the operating cycle by changing the timing of when gas pressure arrives at the bolt carrier group as well as the balance of forward vs rearward force on the bolt during bolt unlocking.


You need a 12"/rifle length to cover a low profile gas block.



More threads for related reading:
http://www.ar15.com/forums/t_3_118/557799_Will_a_carbine_length_gas_system_work_on_a_18_inch_bbl_.html
http://www.ar15.com/forums/t_3_4/554186_what_is_an_intermediate_length__gas_system__mid_length.html

Here is my Noveske SPR with and 11" rail system and intermediate gas system for reference...

By intermediate, do you mean mid-length? Not to hijack but as gas systems go I've always been under the impression that they come in rifle, mid-length (intermediate), carbine, and pistol only.

For the barrel length you are describing, I would recommend the mid-length. Is "intermediate" something between mid-length and rifle? It sounds like you've got a good start already. Which rail systems are you considering?

Thanks for the reference threads. Looks like 12in would be what I'm after. Awesome looking iron! Making me wish my parts would show up. Come on PSA!

Again I'm new to all this, but yes I believe there is a difference between mid length and intermediate length. Somewhere between mid and rifle. As for rail not sure yet but I like the VLTOR and LaRue

The intermediate length gas system is 1.5" shorter than a rifle length...