Hello all. Just joined the forum and this is my first information seeking post. I’m looking to build a 6mm ARC upper to go on my high quality 5.56 multi-cal lower. I spec’ed out and assembled all the parts for my 5.56 build about 15 years ago. So I’m not completely green. I’m looking to build the 6mm upper to be a strict bench rifle and I want to accurately shoot out to 1000 meters and beyond. I know I’ll need new magazines and I’ll have to change out the BCG, or at least the bolt head. There might be some other mods to my lower. But this post is specifically asking about the barrel and gas system. All over the internet, there are so many differing opinions about barrel length, twist rate, rifling and gas system length. I decided to go the round creator and I contacted Hornady. The tech guy said for my application I should get a 24” barrel with a 1:7 twist and a carbine length gas system with an adjustable gas block. I forgot to ask about 4 or 5R rifling. The recommended carbine length gas system seemed a bit short. So here I am asking you folks what you recommend based on your first-hand experience. Please understand I’m not looking for any advice or recommendations on barrel manufacturers or manufacturers of any part for that matter. This is strictly recon on barrel and gas system specs. Once I have gathered enough info and decided on what to get, I will work on who to get parts from. I pre-emptively thank anyone who contributes information to my build!
- Thread starter rcgo88
- Start date
Rifle + 1 gas length for 18-22" barrel. You can probably go rifle + 2 for the 24". I'm sure others will chime in. With the 24" you will gain some velocity for sure, but the round is capable of 1000 yard performance in shorter barrels. I went with a 20" Proof and with factory Hornady 108 ELDm running around 2670. No issue with shooting those to 1k+.
I'm surprised Hornady told you carbine. I know one of their engineers and he told me specifically rifle + 1 when I was seeking parts to put one together. That was before the cartridge was announced to the public. It was well established by them and Proof when the round was developed.
I'm surprised Hornady told you carbine. I know one of their engineers and he told me specifically rifle + 1 when I was seeking parts to put one together. That was before the cartridge was announced to the public. It was well established by them and Proof when the round was developed.
thank you. I was also surprised at the carbine gas recommendation. He did say to go with the 24" so I could shoot the heavier 110 rounds and such. Also, I want to shoot out to a mile. My local club has ranges from 100 meters all the way to 1 mile. Any thought on 4 or 5R rifling?
I find it hard to believe that any (knowledgeable) person would tell you anything shorter than rifle-length with a barrel over 18".
And the barrel length is not (all that) related to the ability to shoot heavier bullets. The twist rate determines that.
I think a mile is going to be a bit of a stretch for the 6 ARC.
Personally, I'd get a 7.5 twist, 20-22" with a Rifle +1 or (maybe +2 if you're going to run a suppressor) and be happy to hit at 1000. Further is certainly possible, but you're going to need largish targets and very little wind.
And the barrel length is not (all that) related to the ability to shoot heavier bullets. The twist rate determines that.
I think a mile is going to be a bit of a stretch for the 6 ARC.
Personally, I'd get a 7.5 twist, 20-22" with a Rifle +1 or (maybe +2 if you're going to run a suppressor) and be happy to hit at 1000. Further is certainly possible, but you're going to need largish targets and very little wind.
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I have never run 4R but have a couple 5R barrels. They shoot fine for me. I'm sure you could make hits at one mile, but there are far better caliber options over a tiny 6mm case. Remember it was designed as a lighter package to mimic or slightly better the 308 in a gas gun.
You are pressure limited in the gas guns, so I don't think you will see much gain going longer than maybe 22". Shooting the 108 or 110 will probably not be much of a difference in downrange performance due to the similar velocity. I could be wrong. I am an A tip fan for my 6.5 and 308 uses. I have no experience with them in the 6 ARC. I think there is a reason they went with 106 for the DOD and LE loads over the 110. They are using potentially shorter barrels too and that could explain why they went slightly lighter. I don't know how much 4 grains will make in velocity between them. The bearing surface is probably different so it can change pressure I would think. I am by no means an expert in bullets or relations of pressure, velocity and bullet shape.
You are pressure limited in the gas guns, so I don't think you will see much gain going longer than maybe 22". Shooting the 108 or 110 will probably not be much of a difference in downrange performance due to the similar velocity. I could be wrong. I am an A tip fan for my 6.5 and 308 uses. I have no experience with them in the 6 ARC. I think there is a reason they went with 106 for the DOD and LE loads over the 110. They are using potentially shorter barrels too and that could explain why they went slightly lighter. I don't know how much 4 grains will make in velocity between them. The bearing surface is probably different so it can change pressure I would think. I am by no means an expert in bullets or relations of pressure, velocity and bullet shape.
6 ARC, 108 ELDM, 2670fps is going to have some issues (in an AR) over 1000yds (about 914 meters).
It's probably trans-sonic @ 1000M.
Hits at a mile?
100F, 25%RH, 28.0InHg, 3000ft elevation, Trans-sonic around 1300yds (about 1200 meters)
It's probably trans-sonic @ 1000M.
Hits at a mile?
100F, 25%RH, 28.0InHg, 3000ft elevation, Trans-sonic around 1300yds (about 1200 meters)
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I know what you mean. I raised my eyebrows when he said carbine. I promise I'm not making that up. The targets beyond 1000 meters are life size animal silhouettes with real time cameras to see hit locations. Wind here in the high desert can be strong but it's mostly calm in the mornings before it gets into the 100 degrees by 11am. I appreciate your input.
Yes, I'm definitely leaning towards a 5R barrel. I know an AR10 would get me to the really long ranges, but I want build off my AR15 lower and see what I can do. Would a 24" barrel hurt performance of smaller bullets, like 90 and 95? Would a 22" barrel give me the best of both worlds for heavy and light bullets? I appreciate it.
I'm not saying your wrong, but I've seen plenty of documented data showing 22" and 24" barrels with a 1:7 twist are more accurate than 18" or 20" barrels shooting at 1000 meter targets. The difference is slight, but there is a noticeable difference IMHO. Of course there are so many other factors to consider. I did see a video of a guy hitting 4" 1000 yard targets with an 20" barrel. I'm just trying to give myself the best chance to be happily successful.
I've hit 935 yards...at night...with a 16" shooting 103 ELD-X (@SoundFx can vouch for that) It's totally doable - this cartridge is like .308WIN ballistics with .223REM recoil. The 26" barrel just surprised me because I couldn't squeeze anywhere near the velocity out of it that I had expected.
26" +2 Bartlein by Craddock
16" Proof carbon
Both are just under 1MOA rifles, but one is far more handy and versatile, while the other spends more time in the safe. A longer barrel usually gives you more velocity, but there's a limit to how long you can go and still see more velocity. With a full power 6mm Creedmoor, for example, that limit is closer to 30" but with an intermediate cartridge like the 6ARC it's probably closer to 20-22".
26" +2 Bartlein by Craddock
16" Proof carbon
Both are just under 1MOA rifles, but one is far more handy and versatile, while the other spends more time in the safe. A longer barrel usually gives you more velocity, but there's a limit to how long you can go and still see more velocity. With a full power 6mm Creedmoor, for example, that limit is closer to 30" but with an intermediate cartridge like the 6ARC it's probably closer to 20-22".
I haven't shot any lighter stuff in my 6 ARC. I have only run the 108 ELDm and have components to load when I finish all of the factory ammo I have on hand.
The barrel length isn't going to affect your downrange accuracy. Sure it can give you some more velocity and help keep your speed up at distance. In the end the ammo, shooter and wind call will be the final for accuracy assuming the rifle is capable of sub MOA accuracy.
The barrel length isn't going to affect your downrange accuracy. Sure it can give you some more velocity and help keep your speed up at distance. In the end the ammo, shooter and wind call will be the final for accuracy assuming the rifle is capable of sub MOA accuracy.
For 6mm ARC I don't think there's any reason to deviate from 1:7.5" twist.
I went 24" Bartlein 5R 1:7.5" with a Rifle+1 tube from CLE before they shut down ( sad face ). It was a bull barrel that was .936" at the gas block forward and somewhere around an inch behind it. Presumably a guy could go 26" or maybe even 28" with a +1 or +2 gas system. I've not done it and can't speak on it for what will work. Adjustable gas is probably a necessity so I'd tend to favor the +1 system because you can always tone it down. I don't know who to go with for barrel supply at this point, CLE was an easy button. I'm sure there are plenty of good smiths out there that can do it, though. Proof Research offers a 22" plug&play option, and I've always had pretty solid luck with Proof. 22" should allow for reasonable success to 1000-1200. Overall I'd say anything in this realm is going to be Rifle length through Rifle+2.
Regardless, 22 ARC is worth considering also. Run with 80-90gr bullets (I'd go 1:7" twist with the 22) it's a better trajectory than the 6mm, but you do lose splash visibility. A little bit of a personal preference all there.
Overall rifling profile is mostly a wash. 3G, 4G, 6G, 5R... I've seen a lot of stuff and most of it works if it's made well. No real obvious trends that have ever stuck out to me. 5R seems to be nicer to jackets for the sake of bullet integrity.
OK just when I'm starting to figure it out here you come. I mean that in the nicest way. I thought I settled on a 1:7 twist and I was leaning towards a 20" barrel. You recommend a 24" with a 1:75 twist. OK stop and hit reset. Then you go and bring up 22 ARC. I never even considered this caliber. Which is more accurate at 1000 meters? Which is more accurate beyond a 1000 meters? Thank you for making me think outside my 6mm box!
Pure hit probability, assuming you can see/record the impacts, I think the 22 has a slight nod on the 6mm. However, if it's a hot miragey day with wet dirt or heavy steel plates it's very easy to miss those little 22 cal bullet splashes.
I don't think you'd really hurt anything with a 1:7" 6mm barrel, either. It's just not necessary. 1:7.5 is going to sufficiently stabilize any bullet that'll fit in the ARC. The 6mm can use all the speed it can get for dedicated 800yd+ that's the reason for the barrel length. It becomes fairly impractical for hunting, but for a bench rifle the weight and length aren't a huge deal. My PRS rifle was 21lb with weights strapped and glued wherever it'd fit.
I don't think you'd really hurt anything with a 1:7" 6mm barrel, either. It's just not necessary. 1:7.5 is going to sufficiently stabilize any bullet that'll fit in the ARC. The 6mm can use all the speed it can get for dedicated 800yd+ that's the reason for the barrel length. It becomes fairly impractical for hunting, but for a bench rifle the weight and length aren't a huge deal. My PRS rifle was 21lb with weights strapped and glued wherever it'd fit.
OK, thank for the info. I have to do more research but I still think I want to build the 6mm upper. My wife and both shoot highly customized 10/22's. So even though a 10/22 obviously won't do much at 1000 meters, the 22 arc seems a bit redundant. Yes, weight is not a problem for this build.
Now when you guys say 1:7.5 is enough, I totally get it. Would a custom made barrel with a 1:7 twist cost more than one with a 1:7.5 twist? I plan on buying all the parts separately and building the upper myself. So if a 1:7 twist barrel cost way more than a 1:7.5, then 1:7.5 it is. But if the cost is negligible, then why not just get the 1:7 twist barrel if it doesn't really matter for accuracy? Rhetorical question, but you get my point.
OK, thank you. Looking at barrels now and yes a 1:7 twist is uncommon. Most are 1:7.5 and I even saw a 1:7.25.
Same I am using a D Wilson in my LMT that is 16" and have no problems going to 1k with factory ammo.
Good Morning All.
I'm leaning towards the X-Caliber barrel over Craddock mainly because of the $285 difference in cost and X-Caliber has many great reviews. Also, X-Caliber has many more configuration options than Craddock. I do have a couple questions though. See them in bold red. I very much appreciate opinions and advice on this configuration.
So I've pretty much settled ( I am open to change) on this X-Caliber barrel:
Steel: 416R SS
Length: 24" - The cost was the same for all lengths up to 18". 22" added $20 and 24" added another $20. Big jump was to 26" which added $100.
Cal & Twist: (6mm) .237/.243 1:7 5R Groove - Twist rates didn't affect cost.
Chamber: 6mm ARC
Gas System: Rifle +2 Choices are rifle or rifle +2. Which is best?
Gas Block Journal Length: 1.92" Choices are 1.00" and 1.92". Not really sure what this even means and which is best?
Gas Port Options: Will Use Adjustable Gas Block. (Slightly Oversized Port)
Contours: AR15 Bull .980, .936GB, .925
Muzzle Thread/Crown Options: 5/8 x 24 Threads (.243 - .358) 11 degree Crown
Fluting: None
Finish: Polish
Cryogenics: Add $100 Is this really worth$100? Research has me leaning towards yes.
Cerakote: None
Logo: None
X-Caliber guarantees sub moa if you use a JP Enterprises or Rubber City Armory BCG. JP doesn't have them in stock and RCA costs $200. If you buy the RCA BCG from X-Caliber, they match it to the barrel and it costs $110. Seems like a no brainer to get the RCA BCG from X-Caliber. Any reason not to?
I'm looking at this adjustable gas block: https://www.jprifles.com/buy.php?item=JPGS-8D Any other suggestions for a high quality GB?
One last note of interest: Aero Precision has a great sale going on right now. Many things marked down 30% - 40%. I bought the enhanced M4E1 Enhanced Upper and Enhanced 15" M-LOK Handguard. These are normally $400. They were on sale for $260. Smokin deal!
I'm leaning towards the X-Caliber barrel over Craddock mainly because of the $285 difference in cost and X-Caliber has many great reviews. Also, X-Caliber has many more configuration options than Craddock. I do have a couple questions though. See them in bold red. I very much appreciate opinions and advice on this configuration.
So I've pretty much settled ( I am open to change) on this X-Caliber barrel:
Steel: 416R SS
Length: 24" - The cost was the same for all lengths up to 18". 22" added $20 and 24" added another $20. Big jump was to 26" which added $100.
Cal & Twist: (6mm) .237/.243 1:7 5R Groove - Twist rates didn't affect cost.
Chamber: 6mm ARC
Gas System: Rifle +2 Choices are rifle or rifle +2. Which is best?
Gas Block Journal Length: 1.92" Choices are 1.00" and 1.92". Not really sure what this even means and which is best?
Gas Port Options: Will Use Adjustable Gas Block. (Slightly Oversized Port)
Contours: AR15 Bull .980, .936GB, .925
Muzzle Thread/Crown Options: 5/8 x 24 Threads (.243 - .358) 11 degree Crown
Fluting: None
Finish: Polish
Cryogenics: Add $100 Is this really worth$100? Research has me leaning towards yes.
Cerakote: None
Logo: None
X-Caliber guarantees sub moa if you use a JP Enterprises or Rubber City Armory BCG. JP doesn't have them in stock and RCA costs $200. If you buy the RCA BCG from X-Caliber, they match it to the barrel and it costs $110. Seems like a no brainer to get the RCA BCG from X-Caliber. Any reason not to?
I'm looking at this adjustable gas block: https://www.jprifles.com/buy.php?item=JPGS-8D Any other suggestions for a high quality GB?
One last note of interest: Aero Precision has a great sale going on right now. Many things marked down 30% - 40%. I bought the enhanced M4E1 Enhanced Upper and Enhanced 15" M-LOK Handguard. These are normally $400. They were on sale for $260. Smokin deal!
Rifle +2 for a 24” barrel
1” gas block journal for your use.
Can’t speak to the cryogenic treatment.
1” gas block journal for your use.
Can’t speak to the cryogenic treatment.
Rifle +2 on a 22” barrel is what a well known builder sold me. I don’t know about “best” but it runs smoothly and reliably and shoots to 1000 easily.
The gas block you choose will determine what length GB journal you need. I’d guess you’ll be using 1” but check the specs on your gas block.
I’ve used cryo on a couple barrels in the past. I’d pass on that unless you have money to burn and you are curious.
The gas block you choose will determine what length GB journal you need. I’d guess you’ll be using 1” but check the specs on your gas block.
I’ve used cryo on a couple barrels in the past. I’d pass on that unless you have money to burn and you are curious.
When I first got into 6ARC I was wondering why several of the manufacturers were doing '+' on the gas system. Eventually after I got everything together I found out why this was the case (at least in my own experience/theory).
6 ARC is very nice, but it's also very gassy. I'm pretty confident that it's just the nature of the cartridge (in general). I opted to 'NOT' go with a '+' length gas system and my rifle was very over gassed, even with an adjustable gas block and a rifle length gas tube. Keep in mind I shoot suppressed pretty much 99% of the time. Ultimately I ended up having to get a much heavier buffer weight in order to get it where I wanted it to be.
The main difference is I have dedicated lowers for everything. I am not swapping uppers around at all. In that instance it is a non issue. On the other hand if you ARE going to swap uppers it would be a pain in the ass to have to tinker with the lower every time you wanted to swap things around. If you are going to swap uppers around you definitely need to be a lot more careful about the gas system length. I would (in that case) probably opt for a 'rifle +1 or +2' gas length with all other things being equal, particularly if shooting with a suppressor.
There is NO WAY in hell I would even consider a carbine length, at least for that application.
Basically with my 6 ARC (24" barrel), with a suppressor the 'tuning process' was pretty much the same as tuning a suppressed shorty 10.5" to get everything running like I wanted. Again because I will not be changing uppers around on various rifles it was a non issue in the big picture. Also if I wasn't shooting suppressed (at all) I could make the regular rifle length work just fine with an adjustable gas block, but as stated before, that (for me) wasn't the case.
As far as twist rate goes, that's another thing I did. I opted for a 1:7 because from the get go I knew I would be primarily shooting heavy for caliber bullets. The thing with 6 ARC is it has a WIDE range of bullet weights that it can shoot. IF you want to shoot lighter bullets you may want to be mindful about twist rates, especially on a long barrel, IE velocity and twist rate determine RPM of the bullet. Again though, in my case I was planning from the outset to mainly shoot the heaviest bullets available. I planned from the start to 'have the option' to shoot those 115 grain bullets if I wanted to, but the opportunity to do so hasn't panned out yet (at the time the component shortage and demand was super high) so I just simply haven't bought any of those bullets yet.
If you do go with a 1:7 (with a longer barrel) you 'might' be chopping off the low end of the bullet weights from the line up depending on how hot you run everything. Anything up to I think 108 will be totally fine in a 1:8 or so barrel. Keep in mind that you should do your own research and figure up the RPMs of various bullet weights at which velocities for yourself. It's been a couple of years since I did it so I am working from a little bit of foggy memory in this post.
If I were going to do it again I would probably opt for an 18" or maybe a 20".
Currently I have a 24" and I think it's a little too long as well. That extra length isn't bringing a whole lot to the party I don't think.
Yes. I have thoughts. Pick one of them and you will be fine.
Absolutely not. 90-95 grain bullets will run no matter any of the nuances we are talking about. Hornady published load data a long time ago starting with something like 58 grain bullets or something like that. The 50 and 60 grain segment of bullet weights MIGHT be compromised if you go with a faster twist on a longer barrel. Beyond that it won't matter.
This is 100% correct. Hornady published data from 58 grains through 110 grains. That is a WIDE range.
At least in theory the faster the twist along side a longer barrel COULD chop off the lower end of that.
Yup, based on my experience now with the other shorter barrels, my initial 26" long range focused build is getting swapped to a 20".
Awesome info! This will be a dedicated bench gun and I will be using my existing Spikes lower and I don't want to mess around with it if possible. I don't plan on shooting suppressed. But I definitely plan on primarily shooting the heavier bullets (105 & 108) as well. X-Caliber doesn't offer a rifle +1 option on the 24" barrel. Seem like a rifle +2 with an adjustable gas block is the way to go for my application. Lots of documented data supporting 24" with a 1:7 twist for the 6mm ARC as the best way to go for consistent long range accuracy. There are plenty folks using shorter barrels with slower twist rates and doing well at 1000 meters. Doesn't mean that is the best way to go for optimal accuracy IMHO. Any thoughts on gas block journal length? Seems like 1" is the popular consensus, although the default for the X-Caliber barrel is 1.92"
Not to derail the thread, but I am wondering what the 'realistic' velocity range someone could get out of an 18" AR platform with 108 grain bullets (all else being the same). As you stated above squeezing the velocity out of a longer barrel doesn't always happen, especially when limited on pressure. I don't like to run super hot loads so on that basis you and I have similar experiences regarding velocity 'advantages'.
I will send you a PM in a minute so please check your in box.
If I were doing it over again I would not go 24" on the length. 20" would probably be the best of all worlds.
The best I could do with 109LRHTs out of the 26" barrel was 2718fps and that was pushing LVR pretty hard. I did get closer to 2750fps but was definitely getting pressure signs. The same load out of a 16" barrel was 2605fps - so barely 100fps lost with 10" less of barrel - that's nuts. So my guess is the ceiling for an 18" barrel with a 108gr bullet is probably close to 2630fps, which is still really damned good performance out of a small frame AR with an intermediate cartridge.
Was this measured in a bolt gun or an AR platform with those barrel lengths and speeds?
I still want to know who at Hornady thought a carbine gas 24" was a good idea. Maybe @Ledzep can get him banned from phone duty.
I was interested because I was originally thinking of a 24" or such build but came across a great deal on a nice one that only has an 18" barrel.
So I wanted to see in the Semi-Auto (AR15 platform) how much of a barrel length could you actually use for improvement before it became a bit moot.
So I wanted to see in the Semi-Auto (AR15 platform) how much of a barrel length could you actually use for improvement before it became a bit moot.
Another D. Wilson converted Proof Carbon 6 ARC user here. I have an 18" with the Rifle +1 and I think it's about perfect for lying out prone and shooting to distance. I wouldn't go any shorter than Rifle length gas system unless you're going to run a 12.5-14.5" barrel. I am getting 1/2" groups with factory Hornaday loads, the 80 VT's and 108 ELD-M's specifically. I have no intentions of hand loading for the 6 ARC.
