6mm ARC Upper Build

[Zen Archery]

I am the same with a 20" proof. It's very temp sensitive, I have seen it change 100fps bases on my conditions

Accuracy was acceptable but 240FPS below advertised velocity using same barrel length was completely unacceptable. Gonna run the 108's through some coyotes hopefully this week.

 

[Zen Archery]

Got these 6mm McNastys loaded up for coyotes and hogs this week. Possibly deer hunting Christmas week.

 

[5.56×250]

Accuracy was acceptable but 240FPS below advertised velocity using same barrel length was completely unacceptable. Gonna run the 108's through some coyotes hopefully this week.

Those velocities are surprisingly low. Compared to the 6.5 grendel , that's velocities seen with 123gr pills . Almost identical powder charges on nearly identical case dimensions. Something isn't adding up , is it .

 

[264MHC]

My friends and I have settled in on using a heavier, thick walled upper made by CMT Tactical.

https://www.cmttac.com/index.php?route=product/product&product_id=125

We have come to the conclusion that it is stiffer that most (if not all) of the other uppers currently offered on the market, and it was recommended to us by the armorer of the US Army's match shooting team. (This is the guy building the rifles for the shooters). So that was good enough for me!

I used a CMT upper and lower for my .243 LBC (Close cousin to the ARC). Mine is the 3A for a side charging BCG. Excellent quality machining and finish. Not too heavy either, even though it is beefed up compared to a standard forged upper. The Port is larger than standard, but I don't think its necessary.

Mine is a 20" BHW barrel 3 Groove 1:8. Shoots everything sub MOA but 95 HVLD's have been the most consistent at .5 MOA. 2830 Avg SD 6 over 28.2 AR Comp in Starline 6.5 G necked down. Killer light recoil deer round. The 95s are really the perfect length and weight for this cartridge. The 105-108 Class just haven't been as consistent and best accuracy was 2650-2680 so the 95 Load beats it anyway. It can go faster with Hodgdon LVR of CFE 223, but AR Comp is so stable, I hate to use anything else.

Be aware the ARC has slightly less case capacity with the shoulder pushed back .030" so my loads may be too hot in an ARC.

 

[Eric H]

I found the bolt gun was very accurate with a 1:7.5 twist. Don't know about the AR.

 

[TRG65]

I built my upper using a heavy billet side charger from brownells, forget the brand. I used a 20" proof barrel. If that is your barrel choice be sure to get a jp bolt. I tried a brownells bolt and it really didn't match up well. Also I tried a jp spring system and light weight bolt that didn't work reliably. Great for 223 not so much for 6ARC. I ended up with a tubbs spring, h1 buffer and a full mass carrier. I'm running a slr adjustable gas block.

Since I played with the buffer system I haven't reset my dies, but with the light system I had to resize my Hornady brass 10 thousandths, with in my opinion is way too much for reliable cycling.

I have not had great luck with my handloads. I'm only getting 2530'ish. That was a node across multiple powders. I managed to push to 2600 but it was showing pressure and over 2600 I popped some primers. In the summer, I saw 2700 with factory, but it was leaving bad ejector marks.

All that said, I like the arc. It does everything I want from an AR better than 223. But it has been very a challenge to get all the bugs worked out for a rifle that can do both some competitions and some hunting. But it and my 6.5Creedmoor AR 10 are the ones that get shot the most.

 

[Son of a Gun!]

I am thinking about building an upper for coyote hunting and am looking for a top-grade upper reciever. I was told to look for one that has a larger opening in the ejection port. Any suggestions about specific one for ARC or port size recommendations? TIA

Aero

 

[jpndave]

Mine is built on an Aero M4E1 upper and lower. Faxon barrel and bolt (I have an Aero as well) BCM rail, lots of goodies in this one. Only been out to sight it in, hopefully schedule and weather will allow some better data/tuning.

 

[quigley257]

I built mine using pretty generic components. I used an EABCO Accuracy 24" 1/7.5 twist barrel and did all the other "accuracy build" tricks(faced the upper, lapped the bolt/lugs, loctited the extension into the upper, etc). I am very pleased with how well it shoots. I have shot nothing but hand loads through it so far and nothing has shot over an inch, with most loads hovering about .5 to .6 for 5 round groups. Needless to say I am happy with the EABCO barrel. I was going to go with the Ballistic Advantage bull barrel, but they were out of stock at the time. I ended up with a rig thats light enough to hunt with and accurate enough to reach out if needed. I'm really liking the cartridge so far and pleased with the accuracy of this build.

 

[264MHC]

I built my upper using a heavy billet side charger from brownells, forget the brand. I used a 20" proof barrel. If that is your barrel choice be sure to get a jp bolt. I tried a brownells bolt and it really didn't match up well. Also I tried a jp spring system and light weight bolt that didn't work reliably. Great for 223 not so much for 6ARC. I ended up with a tubbs spring, h1 buffer and a full mass carrier. I'm running a slr adjustable gas block.

Since I played with the buffer system I haven't reset my dies, but with the light system I had to resize my Hornady brass 10 thousandths, with in my opinion is way too much for reliable cycling.

I have not had great luck with my handloads. I'm only getting 2530'ish. That was a node across multiple powders. I managed to push to 2600 but it was showing pressure and over 2600 I popped some primers. In the summer, I saw 2700 with factory, but it was leaving bad ejector marks.

All that said, I like the arc. It does everything I want from an AR better than 223. But it has been very a challenge to get all the bugs worked out for a rifle that can do both some competitions and some hunting. But it and my 6.5Creedmoor AR 10 are the ones that get shot the most.

+1 On using a full weight carrier and buffer with these cartridges. Same on 6.5 Grendel. I have tried the light buffer route after reading 3 gun forums and such but their goals are very different than ours. Light BCGs and buffers can make for less felt recoil and less muzzle rise but the brass gets destroyed even at lower pressures. Properly adjusting a gas block doesn't seem to help on this front. If you want to run full power (for an AR) loads, and not trash brass, Heavy carriers and buffers help tremendously. I run an A5H1 buffer with a JP polished Rifle length spring, and M16 Cut Young carriers and bolt. I use Superlative arms Adj. Gas blocks. I chose to cut weight in stocks, handguards and to some extent in barrels (no pencil profiles), but maintain full weight in the operating system and receivers. My gun is still only 8lbs Scoped with a 23oz Trijicon 2.5-15.

 

[JustMe2]

I have 3 questions.
1) Does anyone make a 6mmARC upper that is compatible with the original Armalite AR10 .308? Note: Armalite changed their design after they were sold and some parts are not interchangeable with the original, for example barrels.

2) Does anyone make a 6mmARC upper with a charge handle on the left side for a right handed shooter?

3) Can a 6mmARC upper be used on an AR10 lower?

Thanks

 

[264MHC]

6 Arc was specifically designed for the AR15 platform and there is not much to be gained by doing it on the larger AR 10 platform so I doubt you will find parts. I would be looking into something like a 6GT or 6 Creedmoor, or any of the myriad other .473 bolt face short action 6mm cartridges available if wanting to use an AR 10, all of which will out perform the ARC ballistically. The ARC shine because of its relatively great ballistics in the AR15 class of cartridges, while maintaining the smaller, lighter form factor, and low recoil of an AR 15.

 

[264MHC]

If you do go forward with a new upper for your AR 10, not in 6 ARC for reasons stated above, I know Gibbz Arms makes a non reciprocating side charging upper. Not super familiar with the differences between the Armalite and DPMS pattern stuff so I don't know if it will work or not. I do know there seems to be a whole lot more aftermarket support for the DPMS LR 308 stuff so you may have to build a whole new rifle on that standard to get what you want. Or you could just build a whole new AR15 in 6 Arc. Kinda went full circle there.

 

[rpol98]

6 Arc was specifically designed for the AR15 platform and there is not much to be gained by doing it on the larger AR 10 platform so I doubt you will find parts. I would be looking into something like a 6GT or 6 Creedmoor, or any of the myriad other .473 bolt face short action 6mm cartridges available if wanting to use an AR 10, all of which will out perform the ARC ballistically. The ARC shine because of its relatively great ballistics in the AR15 class of cartridges, while maintaining the smaller, lighter form factor, and low recoil of an AR 15.

^^^^ Exactly.

I would only add, that if you're going to step up to the AR-10, you might as well go with 6.5CM.