A "6mm grendel" I think.  Looks like a good lower cost option from an AR platform with just a barrel, bolt, and mag.

I have one and love it.  Essentially it is a 6PPC.

Varget and Berger 105 Hybrids work excellent.  The pic below is 5 shots at 600 yards.  About a 2 inch group.  And a pic of my buddy Sarah MFB shooting it at a match.

If I had to do it over again I might go with a FatRat or Turbo40'

Barrel length and velocity with the 105's?

I've got 3 Grendels and wondering if a 6mm AR is worth it.

Very nice.  What barrel & accuracy?

The Turbo and long barrel should be pretty good at long range.

I have a 6mmAR T40, 28" Krieger barrel I use for across the course and mid range matches.
Very accurate and  easily to load for.  
Nosler 105s, 107s and Hornady Amax all shoot well.
H4895, IMR 4895, and RL 15 have all worked well.  I am going to test some IMR 8082 just for kick and giggles.

22 inch Lilja.  About 2,700 is what I remember.

 Hoser, why would you go with the fatrat or turbo over the standard? I've been wanting a 6mm at for some time now.

The fat rat and turbo 40 are pretty much the same round, just different names.  I am not sure if there is much if any dimensional difference between the two.  It allow for a little more velocity with longer brass life because of the 40 degree ackley shoulder, also may increase reliability of chambering the round.

More case capacity, better shoulder, and higher velocities

http://www.6mmar.com/6mmAR_Turbo_40_Improved.php

If you decide to go this route, go Fat Rat instead of turbo.  You can thank me when it comes time to order a new barrel.

Better feeding is all.

Plenty of accuracy and velocity with the 6AR.

I have a 6mm Mongoose 18inch HB
will be testing it this next weekend

I own a 6mmWOA, it has slightly less velocity than a 6mm Hagar. Both of these wildcats are based on the 6.8SPC case. I actually think they would be better choices IF Lapua brass (in 6.8SPC) were available. As things sit, cartridges based on the excellent 6.5 Grendel brass from Lapua are at an advantage regarding case quality.

I feel that the AR-15 bolt face has too much metal removed when using 7.6x39mm or 6.5 Grendel (.448" bolt face) brass for long term reliability. The 6.8SPC (4.27" bolt face) is better in that regard.

The more I mess with different chamberings in the AR-15 rifle the more I like standard 5.56mm using mid to heavy weight bullets. I feel 5.56mm using the NATO or Wylde chamber, 1/7 to 1/8 twist barrel and 68 to 77 grain bullets is the best all around chamber and ammo for this rifle.

To move up in caliber you really need to develop an entirely new rifle with a longer magazine box and a bigger bolt to do it right. Using the same basic design a great rifle could be developed that would survive long term use and accommodate longer fatter cartridges better that stuffing them to fit in a .223 platform.  

I currently own 6mmWOA, 6.5 Grendel, 6.8SPC and 7.62x39mm AR-15 rifles. I would never trust any of them for self-defense. I have had to perform too many modifications to each of them to get them running reliably to ever trust them with my life.

What Brand Name 6.8 gave you grief?

I just built a 6mm Fat Rat upper with a 22" 8 twist Krieger barrel from Lee Wells at Accurate & Reliable Rifle. It has Lee's "Varmint + .050" chamber to keep the 105 class bullets mag length friendly. Only have about 75 rounds through it so far but the fireforming loads with 30.0 gr of RL15 under 105 Hornady BTHP & Sierra 107 SMKs are averaging a little under 3/4" 5 shot groups at 100 yds. It is a fantastic little round! Hoping to get out this week & put the Magnetospeed on there to get some solid velocity info.  

As stated above, if you're going to do a 6mm then go with the Fat Rat for barrel availability. The 6mm AR Turbo 40 folks seem to only sell complete uppers. I called & spoke to them about purchasing a barrel/bolt/reloading die set combo and never heard back.

My Kotonics 20" H-bar barrel needed the gas port opened up to get it running. All of my C-Products magazines needed to be Dremeled to gain clearance for the ammo to feed into the chamber. Buy PRI 6.8SPC magazines, they come direct from the manufacturer ready to run.

Magazines are the Achilles heal of 6.8SPC and 6.5 Grendel. The cheap ones are crap. The only 6.5 magazines I have used that are any good are the Israeli steel magazines.

My 6mmAR shoots real well.
1/2 MOA using a 22 inch Krieger barrel.

So why is that? I was thinking of trying one of these rounds after i burn out my 6.5 grendel barrel(however many years that takes).

Robert Whitley, who developed the 6mmAR family of cartridges, treats them like a secret family recipe.  He won't sell you a barrel, and he doesn't let anyone else sell them.  If you buy one of his uppers, when you need a new barrel, he will want you to send it back to him for a new one.  I bought one of his uppers in 2011, and have been very happy with it.  It cost a little more than a standard match-type upper, but I was willing to pay a bit of a premium since it was a specialty product.  When it came time to re-barrel, I sent it back to him and paid the premium again (roughly $100 over a similar Bartlein or Kreiger in .223), and I received another great shooting barrel.  When that one was shot out, I called back to get a quote for the next one, and now they were over $300 more than a comparable barrel in another cartridge that I could just change myself.  Screw that.  3 of my local shooting buddies and I who all shoot the Turbo 40º cartridge just pooled our resources and had our own reamers made, and got 8 Krieger barrels set up all at once, with prices back down to reasonable levels.  Now when any of us needs a barrel we just send a blank to the smith and he gets it done for the price of a normal AR barrel.  

The Fat Rat is essentially the same cartridge, but Lee Wells will sell you uppers, chambered barrels, or whatever else you might want to buy, for normal prices.  You can buy the reamers from PT&G or other reamer makers just like any other.  I had no idea the Fat Rat even existed when I bought my Turbo, or I would have definitely gone that route and saved myself some money and hassle.  

Just for background, I am an NRA Highpower shooter, and have shot over 10,000 rounds of this cartridge in competition.  It's a great round.  The 3 barrels I have shot out have all made it past 3,000 rounds before I replaced them.  3,000 rounds is about what I'd shoot in a season, so replacing them at the end is just the practical thing to do, not necessarily a requirement.  The last barrels got changed at 3,600 rounds, and it seemed like it was starting to shoot fewer x's at 600 yards, but hard to be sure.  It still shot well enough to shoot 200's at 600.  I'd say you can count on 3k rounds of top accuracy at those ranges, and wouldn't be surprised to get more.  I have no doubt that I could stick any of the 3 back on and go win matches.  I have also shot it a fair amount at 1000 yards, and if the wind isn't blowing, it can do fairly well there too.  It is definitely out-classed by the bigger 6mm's on up, but it can still hit stuff.

Robert Whitley has barrel/bolt combos listed on his website

We Have Drop-in 6mmAR Barrels and Bolts!

6mmAR Barrel & Bolt Combo - $418.95
Made with premium White Oak Arms match grade barrels, these drop-in 6mmAR barrels are head spaced to our high performance bolts, use a standard rifle length gas tube, and are ported to work with a standard buffer tube.
Only 8 left In stock!
   - 24” White Oak Arms Match Barrel, 1:8” twist, 4 groove, chambered in 6mmAR
             Contour = 1” dia. under hand guard, .875” gas block dia., .850” dia. from gas block to muzzle.
    - Barrel extension w/ M4 feed ramps and locating pin installed
    - 6mmAR High Performance bolt

Turbo 40° Barrel & Bolt Combo - $418.95
Made with premium White Oak Arms match grade barrels, these drop-in 6mmAR Turbo 40° barrels are head spaced to our high performance bolts, use a standard rifle length gas tube, and are ported to work with a standard buffer tube.
Only 4 left in stock!
    - 24"  White Oak Arms Match Barrel, 1:8" twist, 4 groove, chambered in 6mmAR Turbo 40° Improved
             Contour = 1" dia. under hand guard, .875" gas block dia., 850" dia. from gas block to muzzle
    - Barrel extension w/ M4 feed ramps and locating pin installed
    - 6mmAR High Performance Bolt

Link

The kicker is the die sets will run you $200 +

With the fat rat you get a Krieger barrel and bolt for $500 and a whidden die set for $203.
$418 and 200 plus for dies and a barrel.
I'd pick a Krieger barrel over a Wilson.

Well that is definitely new.  Should make his stuff a little more competitive to the Rat.

The Rat or Fat Rat is interesting.

Are these cartridges still around? Seems like the usual suspects for info have disappeared.

Looks like there might be a limited number of stronger Titan bolts available from that 6.8 barrel maker that's banned.

I do not own one but as soon as i find a bunch of new brass  i plan on building one.

I still shoot my Lee Wells built 6mm FatRat with 22" Krieger 1/7.... still fire forming some Lapua brass at have shot 2" at 300 yds with a few rounds.. Varget and 105 Gungnirs(sp).  JP FMOS and SCS and JP adj gas block... shoots and feeds well, quiet with my Thunderbeast Ultra 9