I am looking for a 1/7 twist Heavy barrel in 18 inches to make a MK12 upper. Questions. Who makes the best barrels as far as accuracy? Ammo type 77 grain 5.56 mm. I always hear good things about Noveske so I am thinking of using one of their barrels. I am not into 1/8 twist so that rules out BCM. Daniel Defense sells 18 inch barrels in Mid length. That brings me to another question is it better to run a rifle length gas system or mid length in a 18 inch build. So the questions are as follows, Who makes the best barrels as far as accuracy in 1/7 twist? Does a heavy barrel make a difference? The best gas system for a 18 inch mid or rifle? I will also be running a AAC M4-2000 on this build as well.

MK 12 discussion thread

I've been extremely pleased with my Noveske SPR. It shoots really well with Hornady TAP 75gr 5.56 and 77gr MK262..It actually made me feel as if I had a slight bit of precision/accuracy skills ..

ETA: I'm not a skilled precision shooter by any standards but, I can keep 10 shots right around 1.5" nost of the time and on an extremely rare occasion, tinker with sub-MOA groups using a sled. I typically use front and rear Caldwell bags when I bench this rifle though.

I looked in their I seen a lot of good idea, and good info, but I didnt get all the answers I was looking for.

I hear nothing but excellent things about Noveske barrels. That is why I been seriously thinking of buying one.

Here's some good info from Molon in regards to Noveske barrels..

Noveske Barrel Eval

ETA: While I do not have any personal experience with these other barrels, I have had a few freinds that recommend them so, they may be worth looking at or researching. When I was looking to build an accurate AR based rifle, I ended up with a Noveske but, I considered some of the barrels listed below.

Douglas
Krieger
Lilja
Centurion

I built my MK 12 Mod 0 with a Noveske barrel and couldn't be happier, built a WOA SPR and sold it within a month. My MK 12 "Mod 1" is going to have a Noveske 18 inch barrel as well.

ETA was going to use the swichblock but it wont work with the URX.

Regardless of who does the barrel, you are going to spend some big $$$$ on this project.  After about 18 months of looking here and there for parts and pieces, I broke down and bought my MK 12 Mod 1 upper from High Caliber Sales.  They have several options as to how equipped you would want your upper, but not sure the suppressor you mentioned will work with any of their barrels unless they do 1 just for you.  I am trying to come up with the funds to get the suppressor myself, a Ops Inc 12th model.  Since I only have about 300 yards or so to shoot on, I have a TA-31 ACOG on mine and it does just fine.  Just my opinion...hope it helps!

My Mk12 mod 0 has a Douglas barrel. It is a 1/7 twist stainless barrel. Very accurate with the mk262 ammo from Black Hills. So far the mk262 is the only good ammo I have tried through it. It did shoot some of the Fed m855 very well during brake in and sight in.

High Caliber Sales has the Douglas barrels for sale.

http://www.highcalibersales.com/Site/Available_Items.html

I used a Douglas SPR barrel from Compass Lake Engineering on my Mark 12 Mod1 and it's very accurate. The military version actually uses a Douglas barrel.

I been waiting to here which barrels are used by the military. Finally

That would have been covered in the MK12 thread IIRC. If you would have included that question initially in your post, I'm sure it would have been answered pretty quickly. Also, High Caliber Sales uses Douglas barrels in their MK12 builds so it may be easier to buy a complete upper from them but, that takes the fun out of it..