I'm looking to build a new upper for shooting steel at 1K. I have an Armalite lower and want to make long barreled 308, was thinking 26 inches. Will that length still work with a rifle length gas system?

This is going to be a target gun, I'm not worried about weight. It will be carried to range and that is about all.

I know I can make a 260rem or 6.5CM, but I like the 308 and want to stick to it.

Use a longer than rifle gas system.

op as you already stated their is much better calibers for reaching out that far. I really would suggest rethinking that especially when ammo is the same price.

you can get a 22-24" barrel and its certainly possible to push that 308 at that distance but it takes some work and skill.

good luck

Been there, done that.  I was basically pissing in the wind every time I tried to shoot past about 700yds at sea level.

In the right conditions, especially at altitude, the .308 can stretch its legs nicely with the right load.

The new Tipped Match Kings, Bergers, and bullets like that are great.  New Tipped SMK's

The 155gr TMK has a listed G1 BC of .519, which is awesome really.

The 168gr TMK is .535 G1.

That brings the .308 back into 1000yd territory.

Use a fire control group that has a low mass hammer and full power hammer spring, a heavy high end barrel, and use quality components plus good load procedures, and you're on your way.

If you stick with the 155gr and 168gr, you'll be fine with a RLGS and 22"-24" barrel.  You could go extended length gas system and focus on 175gr, like the new 175gr TMK with .545 BC and run the slower burning powders.  You don't see huge velocity gains going from 22" to 26", as the curve flattens out there.  

Go to Velocity Estimator   to run some comparisons with loads, charge weights, and barrel lengths.  

If you are looking for supersonic out to 1200yds, the newer high BC target pills for .308 are your only real ticket, combined with barrel length.

I didn't realize that Sierra came out with tipped bullets...I was just about to look at the 185gr. Juggernaut.
You think I can just switch over to the 175gr.tmks from by standard RL15 loads which used 175smks?without too much load redevelopment. .

Also...why would u recommend anot extended gas system...couldn't an adjustable gas block compensate for the dwell?

It's not just the dwell, but the pressure.  With the slower burning powders appropriate for longer bullets with lots of bearing surface and weight, the gas port can get hit very hard with pressures that exceed 10,000psi higher than the AR10 was engineered around.

We start to see things like cratered and pierced primers, excessive wear on brass, violent and early extraction, etc.  A lot of people try to address it with adjustable gas, some successfully for the short-term, and others not so much.

I would look at the 155gr, 168gr and 175gr TMK's and compare the drop and drift at 1000yds.  You might find that the speed of the lighter pills with such high BC's will provide a better hit probability.

I have had a lot of good results with the 175gr SMK out of bolt guns from 700-1100yds at higher altitude.  That bullet, when launched from a 1/2 MOA rifle, flies true and doesn't like to be pushed around by the wind much.

I shoot 1,000yards with a 20" barrel DPMS 308AR. in NRA longrange competitions.

We do it with iron sights and appropriate BC bullets.  The preferred bullets are 185 VLD/Juggernauts/Hybrids.  Although i use Hornady 178 HPBT as often as the Juggs.

Don't listen to those that tell you .308 is "marginal" or "not up to it"...It's the indian not the bow.  There are better calibers for this, but the .308 is more than up to the task.

I would simply use a standard rifle length gas system with your 26" barrel.

RRA uses a rifle length system in their 26" 308AR.

An adjustable gas block can regulate gas flow but not dwell or port pressure.

The movement away from 308 comes from a shift in emphasis from shooting KD matches with wind flags to shooting UKD matches under field conditions. Plainly he 308 is marginal for the latter but it has advantages in the former.

How do the Sierra TMKs  feed in a semi? Anyone hear of tips breaking off?

http://www.jprifles.com or Scott @ http://www.medeshafirearms.com

should be able to get you a proper ARMALITE gas tube that's 2" longer than ARMALITE's
rifle length tube.  And get decent adj. gas block while you're at it.

Scott knows the ARMALITE AR-10 from top to bottom.

I'd go:
http://www.grizzly.com/products/7mm-284-5R-1-9-Twist-30-Finished-1-250-Straight-Barrel/T10604
and send it, along with the bolt and extension, to someone that knows the ARMALITE AR-10.

Here's your .308 piece of steel:
http://www.grizzly.com/products/-308-5R-1-10-Twist-30-Finished-1-250-Straight-Barrel/T10731

Here's the extended gas tube I used, but mine's on an 6mm SLR.  It's 2" longer than rifle length.

http://www.whiteoakarmament.com/xcart/product.php?productid=17648