Besides the room for high BC bullets the short fat case is supposedly more efficient for powder burn. In theory a spherical shaped case would provide the best burn efficiency so the next best thing that will actually chamber is a fat, stubby cylinder. This should show up in the velocity per grain from the chrony data.
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6mm ARC Thread
- Thread starter Mountain
- Start date
Besides the room for high BC bullets the short fat case is supposedly more efficient for powder burn. In theory a spherical shaped case would provide the best burn efficiency so the next best thing that will actually chamber is a fat, stubby cylinder. This should show up in the velocity per grain from the chrony data.
just saw this:There was some discussion of this in the Sig Next Generation Squad Weapon, but I did not see an individual thread. I'm going to take a drink of this new Kool Aid flavor. There are a number of articles out there, but this one is a relatively quick read with some comparative 500 yard+ performance vs. the .223 and .308.
Everything You Need To Know About The 6mm ARC
Vs. 308, it has better long range ballistics, less than half the recoil, and is a 30% lighter rifle platform than a 308 AR. What's not to like? LOL I do have to hold my nose a bit as this caliber is a Hornady development and I think Steve Hornady is a jackass.
I plan to borrow the lower from one of my AR15 match rifles and will build an upper with the goal of having a long distance range toy. In the near future I'd like to take some courses at Ridgeline and after trying some 22NRL matches may step up to centerfire PRS matches. I already built an AR 6.5CM and Mrs. Mountain has a nice 308 Aero M5, so those calibers are covered. I thought about a 6CM rebarrel of my 6.5, but 6CM tends to burn through barrels more quickly and is a little hotter than I really need. I can always do that later for relatively short money but I'll probably just stick with the 6.5.
One of the logical digs against the 6 ARC is its lower velocity out of a gas gun. SAAMI specs are 2700 fps for a 108 grain projectile at 52,000 psi. I assume that's a bolt action 24" barrel. Hornady load data lists 52,000 psi as the limit for gas guns yet specs a limit of 62,000 psi for bolt guns. Like the 6.5CM AR I built, I'll use a high pressure JP bolt headspaced to my new barrel which will facilitate slightly hotter loads. Another point of attraction for me is that some of the best heavy projectile 6 ARC loads use Varget and 2520, of which both I use and keep stock in bulk. I'll use one of the better match barrel makers for a 24" (maybe a 22"?) barrel and run a rifle +2 gas system. Might take a couple months before I have my new barrel, but as I have done with prior 'builds' will post targets good or bad along the way.
I realize there are better performance options, but as far as I have seen thus far those options are all in the AR-308 platform. For the cost of a new barrel + bolt, new mags, and not much else we'll see how it goes...
Another concoction from Mountain Accuracy Labs (![[rofl]](/xen/styles/default/xenforo/smilies.vb/013.gif "ROFL [rofl]")
yeah, we'll see...) almost ready to go. Nothing particularly interesting with this 6 ARC gas gun build- just the 4th round of 'Barbie Dress Up' for this particular AR. Previous 3 rounds were for the ever-changing CMP Modern Military match rules. Decided it was time to ditch that last version and will simply use the Bula M14 for that particular match. The low-down on rev 4:
yeah, we'll see...) almost ready to go. Nothing particularly interesting with this 6 ARC gas gun build- just the 4th round of 'Barbie Dress Up' for this particular AR. Previous 3 rounds were for the ever-changing CMP Modern Military match rules. Decided it was time to ditch that last version and will simply use the Bula M14 for that particular match. The low-down on rev 4:- I found a BA 24" barrel on sale relatively cheap to use while I wait for the Compass Lake Bartlein barrel, which will have a +2 rifle gas system. The standard gas system on the current BA barrel looks nice, with the gas block tucked away inside the hand guard, but I think I'll be bleeding off some velocity with the port so far away from the muzzle.
- BA barrel is a heavy profile- .936" at the gas block and really no taper to the muzzle. Heavy A-F. I had a Luth AR butt stock and added Luth's weight to it to help balance out the porker barrel.
- I pulled a Geissele SSA-E from another AR but may try a Trigger Tech if they have one of those sales everyone mentions.
- Bolt is the Wilson Combat that I mentioned in an earlier post. I don't recommend playing with hotter loads without this bolt or the JP high pressure bolt.
- I don't like the position of the rings for the scope and may steal the offset mount off of my service rifle.
- I was curious to know if I'd like the vertical grips that are often seen on PRS bolt guns. The MPA grips aren't particularly expensive and will fit an AR, so why not give it a try. I did have to drill a hole for the detent and spring as the grip does not come with that provision.
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OK, first range report. While I don't think it is yet time to wear a bag over my head it's definitely not time to strut around like Connor McGregor. The rifle ran perfectly. No hitches and it sent the brass more or less into the same spot at about 4:00. Very low recoil. Even at maximum scope magnification, it was easy to keep the impacts in view.
I only have the Hornady factory 108 ELD's to shoot. I ran a few 5-shot series with the MagnetoSpeed chrono, so here's how they did:
In general they had a flyer that would ruin an otherwise decent group. I was single loading for the groups and the flyer wasn't always the same in the sequence. Groups were ~1-1/4" to ~1-1/3". Without the flyer, one of the groups would be sub half MOA which would be great. I lapped the receiver face & the barrel was properly torqued, so I don't think that's an issue. Everything else is tight as a drum and the scope is decent (NightForce NX8).
I'm going to give the barrel a good cleaning and some light lapping with an extremely fine diamond compound that I have. That worked well for my 6.5 CM barrel. This barrel is a BA that I bought on sale from Classic. It was very dirty when new. That could be manufacturing residue or perhaps Sweaty Ben re-sold a used returned barrel as new? I did have a crosswind that was gusting to 15 mph, but I was pretty careful to work around the gusts. The wind and flyers were from right to left, so maybe that was the issue. Or possibly a booger hook mistake?
Might take me a while but I'll work up some hand loads with Berger 105's and 108's. This is a 1:7 twist barrel so I think it should favor the 108's but we'll see.
I only have the Hornady factory 108 ELD's to shoot. I ran a few 5-shot series with the MagnetoSpeed chrono, so here's how they did:
- Min 2612 fps
- Max 2632 fps
- Avg 2621 fps
- SD = 7.8
- ES = 20
In general they had a flyer that would ruin an otherwise decent group. I was single loading for the groups and the flyer wasn't always the same in the sequence. Groups were ~1-1/4" to ~1-1/3". Without the flyer, one of the groups would be sub half MOA which would be great. I lapped the receiver face & the barrel was properly torqued, so I don't think that's an issue. Everything else is tight as a drum and the scope is decent (NightForce NX8).
I'm going to give the barrel a good cleaning and some light lapping with an extremely fine diamond compound that I have. That worked well for my 6.5 CM barrel. This barrel is a BA that I bought on sale from Classic. It was very dirty when new. That could be manufacturing residue or perhaps Sweaty Ben re-sold a used returned barrel as new? I did have a crosswind that was gusting to 15 mph, but I was pretty careful to work around the gusts. The wind and flyers were from right to left, so maybe that was the issue. Or possibly a booger hook mistake?
Might take me a while but I'll work up some hand loads with Berger 105's and 108's. This is a 1:7 twist barrel so I think it should favor the 108's but we'll see.
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i my 6 dasher hornady eld did not perform well at all.
try noslers and bergers. start from 105 bergers hybrid, as that is what most people use.
and - yes on both things - lapping the upper is mandatory and you absolutely have to clean it many times from the gunk it comes with.
i have no idea what speed is it suposed to be in a gas gun for 6arc, my 105gr berger load goes at 2945fps, in that 6 dasher.
try noslers and bergers. start from 105 bergers hybrid, as that is what most people use.
and - yes on both things - lapping the upper is mandatory and you absolutely have to clean it many times from the gunk it comes with.
i have no idea what speed is it suposed to be in a gas gun for 6arc, my 105gr berger load goes at 2945fps, in that 6 dasher.
I don't think the SDs were too bad for factory. I believe you'll get a bit better results from your handloads. I've read about people finding the chamber jump to be a bit much and custom ordering barrels with the chamber set back a bit. It sounds like you have some room to play with if your OAL is a bit less than magazine length. So, you'll be able to account for some of that.
Curious what made you opt for the +2 gas system? Thats something I haven't played around with yet and l was just wondering your thoughts on it.
Curious what made you opt for the +2 gas system? Thats something I haven't played around with yet and l was just wondering your thoughts on it.
IMHO the standard rifle length gas system wastes the energy that could otherwise be harnessed by a 24" barrel via placing the gas port that much shorter than a +2. That's my theory- let the projectile benefit from more barrel length / more propellant expansion before the port bleeds off energy. I was thinking this plus the Compass Lake folks recommended +2 with a 24" barrel, so that settled the issue. The Ballistic Advantage barrel I have now is a standard rifle length, so once the Bartlein +2 is finished by Compass Lake I'll be able to compare velocity.
Agreed the SD's are not that bad, especially from a gas gun. I have a prepared case and can check the jump if I pull a bullet. Perhaps it is excessive. It is nice to have some extra room in the mag to work with that, which is one of the design features of this cartridge: short OAL even with the longer, higher BC projectiles. Excessive jump might explain getting flyers despite the relatively tight SD's.
At the risk of sounding cocky, I've built several sub-MOA AR's and I'm not seeing any mechanical build related issues with this build that might detract from accuracy. I've shot ~1/2 MOA groups with this lower and a tight chambered .223 upper. Hopefully the issue is the ammo, otherwise the barrel might not be that great. Possibly a combo of the two. For a semi-auto the random nature of the flyers often indicates some sort of mechanical issue. If not that, something is off with the loads. If the barrel were a problem I'd expect generally bad groups, not generally very tight groups with some random flyers. We'll see- probably will be a week or two before I have the opportunity to sort this out.
I also share your opinion/suspicion that a longer length gas system contributes to more velocity. And I think that was a good idea for this cartridge. Especially since you're going with a very long barrel to give it that extra push, might as well do what else you can to help out. Are you using an adjustable gas block? i find that in other cartridges I've gained a bit by closing off as much gas as possible. I think adding length to the gas system sort of accomplishes the same thing in a different manner.
In my early research on the cartridge I was finding that factory loads were easily beat by handloads in terms of accuracy, but the trade off was velocity (with published max loads). So I think between powder choice and OAL you have a whole lot to work with to tighten those groups up. The one on the right looks pretty decent without the flyer, as it is. Thanks for the updates!
In my early research on the cartridge I was finding that factory loads were easily beat by handloads in terms of accuracy, but the trade off was velocity (with published max loads). So I think between powder choice and OAL you have a whole lot to work with to tighten those groups up. The one on the right looks pretty decent without the flyer, as it is. Thanks for the updates!
I have an adjustable gas block on the way, so I'll see what that does. My thoughts were the same- only flow enough gas to function reliably. I'm a little disappointed with the first shots on paper, but I have some levers to pull that could net some tighter groups.
Somewhere in my copious rambling I was bitching about Hornady's ridiculously sh!tty packaging for this ammo and how the tips were free to bang around on the bottom of the box. Well...
That pic actually didn't catch the full magnitude of the bent tip. There are a few of these in the box. Maybe I can shoot in a sideways curved trajectory like Angelina Jolie's character in the movie Wanted? Oh wait, apparently some of my shots did that.
Great job, Steve Hornady. Unprecedented volume the last couple years and you're pinching pennies on packaging. Douche.
That pic actually didn't catch the full magnitude of the bent tip. There are a few of these in the box. Maybe I can shoot in a sideways curved trajectory like Angelina Jolie's character in the movie Wanted? Oh wait, apparently some of my shots did that.
Great job, Steve Hornady. Unprecedented volume the last couple years and you're pinching pennies on packaging. Douche.
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i dunno, i do not have much love for those eldx bullets. the one and only caliber where it seems to work for me is the 6.5CM, but even there bergers performed better.
but they just cost much more than - almost twice as much as eldx, and benefit is quite ephemeral.
in all other calibers i have - .223, 6, .308 - nowhere those eldx worked well. .223 was especially bad, and i permanently changed to noslers there, preferring RDF ones. but CC also works fine.
in 6mm bergers hybrids are undisputed bests, it seems. noslers did not group as good as bergers, so, i just bought 2K of those bergers once and now use them slowly.
Same issue? ELD nose?
I had good results from 123 ELD's with the 6.5 Manbun, but only at 100 yards. At 300 yards the 140 RDF's were better and that's what I used on plates out to 1100 yards. The best factory ammo was Winchester Match 140's which I assume had 140 SMK's. That stuff would shoot bug holes at 100 which is awesome for an AR.
my theory is that all eldeses are very sensitive to the amount of jump, and to make them work well in gas guns due to that is a challenge, as you sitting them to fit the mag length.
with bolt guns and much longer magazines it is a way less of an issue.
one way or another - i was just not that interested to deal with them, as there are plenty of alternatives out there.
Some new data to share after pulling a projectile to check 'jump'.
Hornady Factory Match Ammo 108 ELD
Measured 29.98 grains of a fine spherical powder- let's call it 30.0 grains and probably Leverevolution powder.
OAL is 2.250"
Case length 1.484"
Projectile length 1.257"
Projectile weight was right on the money, 108.0 grains
Hornady Gas Gun load data doesn't even list a load for 108, and if it did 30.0 grains would probably be slightly over the limit for the 52,000 psi. No matter, I have a top quality bolt and did not see any signs of over pressure on the spent cases nor primers. Maybe there is fine print on the box that says bolt guns only? LOL if not, Hornady isn't paying attention to their own 'rules'. That said, use this ammo in a gas gun at your own risk. I am NOT saying it is OK.
The 108 ammo leaves plenty of room in a magazine. I have two different makes of 6.5 Grendel mags (what you use for 6mm ARC) and both have ~2.42" of cartridge length clearance.
Yep, this is a very short cartridge for the length of projectile. If anyone is using a Hornady OAL gauge you will find that the length of the section that will be inside the prepared cartridge is too long for the longer projectiles. I had to trim it with a tubing cutter to allow seating the bullet deep enough in the prepared cartridge.
I'll remeasure to be sure but it looks like this particular ammo at 2.250" OAL provides a jump of 0.060" in the BA barrel. A bit generous and I'm sure I'll need to tighten that up for better accuracy.
I don't know if anyone reading this has experience with Leverevolution powder but if that's what's in this ammo, it runs very dirty and the residue isn't very easy to clean compared to my usual match powders (Varget, Benchmark, and 2520).
.
Hornady Factory Match Ammo 108 ELD
Measured 29.98 grains of a fine spherical powder- let's call it 30.0 grains and probably Leverevolution powder.
OAL is 2.250"
Case length 1.484"
Projectile length 1.257"
Projectile weight was right on the money, 108.0 grains
Hornady Gas Gun load data doesn't even list a load for 108, and if it did 30.0 grains would probably be slightly over the limit for the 52,000 psi. No matter, I have a top quality bolt and did not see any signs of over pressure on the spent cases nor primers. Maybe there is fine print on the box that says bolt guns only? LOL if not, Hornady isn't paying attention to their own 'rules'. That said, use this ammo in a gas gun at your own risk. I am NOT saying it is OK.
The 108 ammo leaves plenty of room in a magazine. I have two different makes of 6.5 Grendel mags (what you use for 6mm ARC) and both have ~2.42" of cartridge length clearance.
Yep, this is a very short cartridge for the length of projectile. If anyone is using a Hornady OAL gauge you will find that the length of the section that will be inside the prepared cartridge is too long for the longer projectiles. I had to trim it with a tubing cutter to allow seating the bullet deep enough in the prepared cartridge.
I'll remeasure to be sure but it looks like this particular ammo at 2.250" OAL provides a jump of 0.060" in the BA barrel. A bit generous and I'm sure I'll need to tighten that up for better accuracy.
I don't know if anyone reading this has experience with Leverevolution powder but if that's what's in this ammo, it runs very dirty and the residue isn't very easy to clean compared to my usual match powders (Varget, Benchmark, and 2520).
.
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Now an update on the build.
That damn Hornady ammo was very dirty which required some work to clean. Once I had the barrel clean I gave it several passes of JB bore paste. Even when first cleaned as new, it was tough to push a patch through the barrel. Now that I've 'lapped' it with the JB, patches run very smoothly through the barrel. I've used a very fine micron size diamond compound for this in the past, but in this case I think the bore 'feels' good with just the JB. I know there is much debate about new barrel care, but this is part of my process and it helped me to get sub-half MOA groups from a 6.5CM AR.
One of the problems with a standard length rifle gas system on a 24" AR barrel is that there is too much length after the port. This generates much higher pressure against the BCG than say for an 18" or even 20" barrel with the same standard rifle length gas system. I've been building my own AR's for years and in several different calibers, but this is the first time I am going to an adjustable gas block. The gas block I was using was a cheap .936" one I had in the parts pile and upon its removal I can see that it didn't fit well and leaked a lot of gas from various points. I just replaced it with an adjustable unit from Aero, which fit much more tightly.
I've had very good results from RCBS Matchmaster dies and have a set on the way direct from RCBS. I also ordered a couple different neck bushing sizes to try. I probably won't have my own loads ready next trip to the range but I'll have a chance to see whether or not the adjustable and non-loose fitting gas block plus some bore polishing will work to tighten groups.
That damn Hornady ammo was very dirty which required some work to clean. Once I had the barrel clean I gave it several passes of JB bore paste. Even when first cleaned as new, it was tough to push a patch through the barrel. Now that I've 'lapped' it with the JB, patches run very smoothly through the barrel. I've used a very fine micron size diamond compound for this in the past, but in this case I think the bore 'feels' good with just the JB. I know there is much debate about new barrel care, but this is part of my process and it helped me to get sub-half MOA groups from a 6.5CM AR.
One of the problems with a standard length rifle gas system on a 24" AR barrel is that there is too much length after the port. This generates much higher pressure against the BCG than say for an 18" or even 20" barrel with the same standard rifle length gas system. I've been building my own AR's for years and in several different calibers, but this is the first time I am going to an adjustable gas block. The gas block I was using was a cheap .936" one I had in the parts pile and upon its removal I can see that it didn't fit well and leaked a lot of gas from various points. I just replaced it with an adjustable unit from Aero, which fit much more tightly.
I've had very good results from RCBS Matchmaster dies and have a set on the way direct from RCBS. I also ordered a couple different neck bushing sizes to try. I probably won't have my own loads ready next trip to the range but I'll have a chance to see whether or not the adjustable and non-loose fitting gas block plus some bore polishing will work to tighten groups.
Hornady's 11th edition lists the 108 ELD Match gas gun loads on page 244. Lever is listed with a max load of 29.7 grains (C.O.L. of 2.245") at a velocity of 2575 fps from an 18" barrel. That and CFE 223 are the two powders that produce the highest velocity accoding to Hornady's gas gun data for the 108s.
