I realize this thread is a few years old but unless I missed someone I'm adding the only input on a 12.5" Lilja barrel in 6mm ARC from Six Sigma.
I'd like to think I had the original 12.5" 6mm ARC idea, I bought the barrel early but didn't put it to use until this last year so second is good enough ..
I've been shooting AR's since the early 90's and have built more than I can remember, but I do remember that I consistently learned more with each build about what makes one shoot and perform to it's potential. Easy to make one cycle reliably and be "accurate", quite a different thing to lift it to its full potential. My barrel was from Six Sigma, at a time when they were difficult to find, the person who helpped me there was nice, walked back to see if one was on the shelf like the computer was telling him, he answered my questions and it shipped that day.
This build gave me no trouble at all. From the first round it was 100% reliable. Initually I used a AGB w/A5 buffer setup with a springco green rifle spring and a A5H2 buffer, and using Hornady 105g Black, it ejected perfectly at 90 degrees from the first shot. That night when cleaning it at home I changed out the AGB with a regular gas block and it's ran reliably since. I fully followed Lilja's break in procedure and took my time. Does it make a difference? Maybe. If I produced barrels I'd probably have an opinion worth reading. Nothing special or off the wall parts were used. I just used quality parts that I've come to believe are great parts. A recent addition to my personal builds are the Geissele "posi-snap" selectors. With just a touch of polishing they work just like I like them to work. The BCG was a spaced JP bolt w/3 gas rings from Sprinco and their ejector and extractor springs mated to a Young's National Match enhanced carrier. I wish I knew where to still get those, that was my last one. Using a Leupold Mark 6/H59 with a Reptilia AUS and a DPP on a 45.
I did Cerakote the barrel using their V-169. This finish is very thin and if the barrel is properly prepped will not come off after baking. Its' good to 1800f.
Weight with a Blue Force sling, optics, and SBA3 brace is right at 8lbs.
From my first 10 groups of five, using Hornady 105g Black, average mz was 2430fps (using a Garmin Xero), 4 groups were under .5moa, 3 were under .75moa, 2 under 1moa, and 1 was 1.2moa. Keeping a proper hold when you stop to clean after each round is bit of an issue. Now after 200 rounds mz has settled in at 2470 (same lot of ammo) and I haven't shot paper since the first 50, but I routinely hit MOA steels out to 700m.
Really, this is a very capable build. I just wish as many people used 6mm ARC as do 5.56mm.
