I stand corrected. Just looked at Shilen website, they offer a 7.5 twist in a 4 groove......probably the cats meow for 6arc
6MM ARC Help !!
- Thread starter Benhonda1
- Start date
Here is the BCM side by side Mk2 vs mil spec. Some billet receivers JP and Iron City especially have an even more material the length of the upper. BCM made the MK2 as an improvement but with minimal extra weight. Thermal fit is a more solid design watch Joe Carlos explain the importance of fitment. Longer heavier barrels benefit from a stiffer receiver. How much exactly, I’ve never seen a test moving a barrel from receiver to receiver to see the difference. If you’re buying new you might as well get those features provided your not pack hunting with the gun. I used to be a regular bolt action accuracy guy and had Colt AR to blast with and shot some high power with open sights. Then I started working with some serious AR guys and shot a JP AR-10 .308 that will shoot right with all but the most accurate bolt guns. After shooting ragged hole after ragged hole with that JP I started building AR’s. Done right with careful attention to the parts the results are amazing.
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I’m going to let Mike choose a twist based on his experience with customers. 4 groove Ratchet cut 1 in 7.5 was what I was thinking though. I’ll have him chamber it to my bolt, assembling the upper is cold weather garage fun.
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More brain food:
Calibers and Rifling | Bartlein Barrels
www.bartleinbarrels.com
The Tack-Driving AR — Secrets to AR Platform Accuracy « Daily Bulletin
One Shooters' Forum member asked: What makes an AR accurate? What parts on an AR can really affect accuracy -- such as free-floating handguards, barrels, bolts, bolt carriers? He wanted an honest, well-informed answer, not just sales pitches.
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If you have another ar that works, and can donate more standard parts, start at the back and work forward till you find the offending pos.
One of the parts you have on it sucks. I don't gaf what brand they are.
An h2 buffer and +10% flat wire spring will probably take care of it.
One of the parts you have on it sucks. I don't gaf what brand they are.
An h2 buffer and +10% flat wire spring will probably take care of it.
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From reading @Frank Green comments on the subject im not so sure it does . I have four different caliber ratchet groove barrels ,all group small ,clean easy ,and reached my targeted fps with below max book charges . I'm a fan I guess.....if it ain't broke,don't fix it kinda deal for me .
ETA: I spoke to dtech right after the 6 arc came out and they were waiting for a reamer ,priced a build like I wanted, round numbers ,$1000. Just never got around to ordering one. Going back n forth trying to decide bolt or gas......I'm going bolt when I buy, but if I'd gone gas, I'd gladly wrote a check to dtech .
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Prior to Aug. 2022 I did say that for the most part the style and number of grooves has no bearing on accuracy and or barrel life. There was one exception and that was with 6mm and in guns like the 6PPC benchrest guns. Those short jacket and it's worse with the boat tail bullets in the 65-68gr class weight of bullets that they don't like the 5R rifling. The best you will get the bullets to shoot is upper .2xx's to flat .3xx's. Now is that bad for accuracy? Shooting BR where if your not shooting in the upper .1xx's to flat .2xx's that isn't competitive enough. That was the only exception.
About 3-5 years prior to Aug. 2022 One ammo/bullet maker and the Army did testing the same year and independently of one another. Both calibers where 6.5mm. So the bullet maker tested by accident a 4 groove barrel and a barrel with 5R rifling. Again the test was by accident. Testing was done at 1k yards. Army did testing out to 1200 if I recall correctly. What they found was that at extended ranges the bullets fired out of the 5R rifling flew more consistently/accurately if you will out of the 5R rifled barrels vs the 4 groove but nobody knew why or could figure it out.
Which brings us to.....
Until Aug. 2022 a bullet maker did testing for a gun manufacturer.... a dozen test barrels where used in the test. Unknown to me we made all the test barrels. Seven of the barrels had saami spec. 6 groove rifling and five of the barrels had 5R style rifling. Accuracy testing for group measurements was done at 200 yards. All indoor shooting. 320 rounds fired all groups where 10 shot groups for accuracy. Every bullet fired (think there where like 4 different types of bullets shot) where photographed right after the bullet left the barrel. Took like two days to set up for the test and the first round of testing they shot 160 bullets and took like 11 hours. They repeated the test 2 weeks later to make sure none of the numbers where skewed and to see if the results duplicated and they duplicated exactly.
In the grand scheme of things in the Aug test the bullets fired out of the 5R rifling only shot like a grand agg of the groups combined like .100" smaller than the groups out of the 6 groove barrels. So one could say not very significant.... but! What was found in the pictures was every bullet fired out of the 6 groove rifling had a burr on the side of the bullet. I asked how big was that burr? They said as big as a 1/32" of an inch. I then asked... did that affect the bc of the bullet? They said yes by as much as 5%. I said if they do that test ever again.... run it at 500 yards or further.... that 5% affect on the bullet will now show up even more on the target further down range.
So this Aug test in 2022 now backs up what the other ammo/bullet maker and the Army seen a few years before but didn't know why or what they were seeing.
Obermeyer always claimed that bullets fired out of the 5R rifling was more accurate but he never had any real data that I know of to back up that claim. I do believe that it helped fight bullet failure but after that Aug 2022 test.... now there is data/proof to back it all up.
Short range...again you might not see a big difference.... but long range is a different story. You have a lot of things going on at longer distances that can wreck your accuracy... if the 5R rifling is helping the consistency of the bc of the bullet... that is significant.
Later, Frank
Is it the number of lands/grooves (odd vs even), or the design of the lands/grooves?
It’s the change in shape that the rifling makes to the bullet shank. If there are sharp angles like with traditional rifling, the bullet you see before you loaded it, is not the bullet that exits the muzzle. It has had tens of thousands of psi generated behind it, then forced into a tube with spiral grooves that create a lot of shearing force to the jacket and torsion to the jacket and core.
5R still changes the bullet shank shape but with less-drastic deformations.
Since the projectiles are spinning in the 150,000-280,000 rpm range generally, any deformations to the shank will affect gyroscopic and aerodynamic performance negatively.
Sounds like there could possibly be a more pronounced effect with faster twist barrels?
Both... the odd number of lands and grooves are working together in my opinion. The lands don't directly oppose one another with the odd groove barrel in this case the 5R (you don't get the same benefit like with a 3 groove even though it's a odd number) so it will distort and upset the bullet jacket less in conjunction with the 5R rifling the 5R rifling vs conventional rifling where the lands in the conventional rifling meet the grooves at a 90 degree edge.... it's both of those things with the 5R rifling that is helping not to create the burr on the bullets.
When the bullet is fired and making the jump into the rifling you have to picture that is basically getting pounded into the rifling with 60k+ pounds of pressure behind the bullet. The bullet to some extent will swell up/obturate when it enters the rifling. I've asked the question and no one can measure it or knows how to measure it but by one experts opinion from what he sees with bullets is they expand at approx. a .001" in diameter and then have to squish back down to the bore/groove size.
It wasn't just match bullets they tested either, as they also tested solids and I believe high end hunting bullets etc... they tested like 4 different bullets and again they tested a total of 320 rounds/bullets where fired. Everything repeated and confirmed what they seen in the first round of testing.
I'd like to know what that test cost to do? Not just the 12 barrels but all the gunsmithing time... the set up time... taking pictures of 320 bullets, loading, the equipment, recording all the data etc... I asked but never got a response. LOL!
The rpm of the bullets in the test was in the 308k rpm range according to my numbers.
My guess/opinion is no. Strictly due to the style and number of grooves. I think velocity and pressure will make things worse though.
Why do I say no. BR guys have seen burrs like on the boattails where you get the transformation from the main diameter to the boattail. You see the burr or smear there as well.
Does this hold true for gain twist since it is effectively continuously imprinting the entire path of the rifling?
I’ll say yes. I haven’t seen anything to date to say no.
Thanks for the post. It sounds to me like I’ll be going for the 5 groove. I’ve been in this forum for a little over a month and learned more it that time than I have in the previous 5 years! Much appreciated!
My Odin 22" +2 barrel has a .101 gas port, that is a rather large port. My gas block is barely open and I run a rifle buffer with 4 of the steel weight replaced with aluminum weights and 10% reduced buffer spring. Rifle barely gets dirty because the gas is turned down so low, it probably stays locked up longer as well. Sub .75 moa gun..
I have run my 20" Proof with a regular AR rifle buffer and spring, then changed to the A5 with the similar weight buffer as the AR rifle. Sorry I forget if it's H2 or H3. Its around 5.3ish ounces or something close to that. No issues and I am running the Aero adjustable gas block with a full JP carrier and bolt. I ran it for a while with the one piece gas ring and changed to regular rings after 50 rounds or so. Oddly I had to click the gas block a couple more to the open side with the 3 ring set up. I run the ASC and Duramags with factory Hornady 108 ELDM ammo only so far. Zero issues getting it to lock back or cycle. It's barely restricted gas wise now, but it's not wide open.
My buddy has run those Armaspec buffer setups and he always complains that they feel like they have a heavier spring than they should have. He has had no issues with the JP captured spring running the same weight. Maybe try a different buffer setup or change the springs if you can.
My buddy has run those Armaspec buffer setups and he always complains that they feel like they have a heavier spring than they should have. He has had no issues with the JP captured spring running the same weight. Maybe try a different buffer setup or change the springs if you can.
I had same issue on my 6mm Grendel and fixed it by enlarging the gas port. All alignments were confirmed.
...didn't see a final answer from the OP on if he resolved his issue or not. A couple thoughts to throw out to those planning a build:
1. If one starts with an adjustable gas block (AGB), use a standard buffer system to start. The AGB's function is to moderate the gas flow to control any potential "over gassing". Using an AGB and heavier buffer system can introduce problems that are harder to pin down the actual source. and if it is under gassing or over gassing...the symptoms are the same.
2. For the 1st firing, load only a single round in the magazine, fire it and check if you get lock back. Depending on result, adjust AGB accordingly, load 1 round in mag, fire, check for lock back. Repeat this process until lock back is achieved. Once achieved, load 2 rounds in mag, fire and see if it picks up second round and fires it followed by lock back. Adjust accordingly (usually open to next adjustment available) untill feed-fire-feed-fire-lock back is achieved.
Caveats:
a. Use the AGB's manufacturer recommended "starting" setting. Some recommend fully closed and opening each time until lock back, others the reverse. I typically start my SLR's at full open for the 1st round to blow everything out of the gas system, then close to #3 open to begin the testing.
b. Use the ammo (factory or handload) you anticipate using the most often to establish your baseline, adjust accordingly when you settle on the ammo/load.
I run SLR' AGB's on all of my rigs (10.1" 300BO, PLGS) (16"CLGS- 300BO, 223W/556, 308) (18" RLGS 6ARC) and standard carbine buffer systems. All function 100% reliably., with ejection patterns being @3:30-4:30. I can't run a can, not legal in my locale, so no info on that.
I don't worry about the AGB seizing and locking, after shooting I'll adjust the screw completely closed, completely open a few times to remove the carbon accumulation. Never had a problem with seizing. If the particular rig has been stored and idle for a while, I'll do same before heading to range/field so I don't have any surprises. Standard PMCS... ;-)
YMMV
1. If one starts with an adjustable gas block (AGB), use a standard buffer system to start. The AGB's function is to moderate the gas flow to control any potential "over gassing". Using an AGB and heavier buffer system can introduce problems that are harder to pin down the actual source. and if it is under gassing or over gassing...the symptoms are the same.
2. For the 1st firing, load only a single round in the magazine, fire it and check if you get lock back. Depending on result, adjust AGB accordingly, load 1 round in mag, fire, check for lock back. Repeat this process until lock back is achieved. Once achieved, load 2 rounds in mag, fire and see if it picks up second round and fires it followed by lock back. Adjust accordingly (usually open to next adjustment available) untill feed-fire-feed-fire-lock back is achieved.
Caveats:
a. Use the AGB's manufacturer recommended "starting" setting. Some recommend fully closed and opening each time until lock back, others the reverse. I typically start my SLR's at full open for the 1st round to blow everything out of the gas system, then close to #3 open to begin the testing.
b. Use the ammo (factory or handload) you anticipate using the most often to establish your baseline, adjust accordingly when you settle on the ammo/load.
I run SLR' AGB's on all of my rigs (10.1" 300BO, PLGS) (16"CLGS- 300BO, 223W/556, 308) (18" RLGS 6ARC) and standard carbine buffer systems. All function 100% reliably., with ejection patterns being @3:30-4:30. I can't run a can, not legal in my locale, so no info on that.
I don't worry about the AGB seizing and locking, after shooting I'll adjust the screw completely closed, completely open a few times to remove the carbon accumulation. Never had a problem with seizing. If the particular rig has been stored and idle for a while, I'll do same before heading to range/field so I don't have any surprises. Standard PMCS... ;-)
YMMV
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Made decisions today. Going with aero advanced upper and lower with 16.5” hand guard. Hart 22” barrel with 7.5 twist 5R rifling. Haven’t decided on BCG just yet but once I do will send upper and BCG to Hart. They will (for lack of knowledgeable term) match the bolt and barrel and set head spacing and assemble barrel and upper. Only issue is they are 4 months out
but that gives me time to get everything else decided on and purchased, including ammo and reloading supply!I’m in the middle of building a new gas gun in 6mmARC. I’ve currently got a Hart barrel ordered. 24” medium heavy. They recommend a regular rifle length gas system. Most of my research tells me I should go with rifle +1 gas system. What are your thoughts and suggestions?
Rifle gas is good on a 6.5 Grendel. +2 gas works on a 224 predator (22arc) but I kinda wish I had gone rifle gas. Ejection can be anemic on lighter loads. That brackets 6 arc, and I’d surmise either rifle or +1 would be fine…
That’s my feeling as well. The tech at Hart told me they run RLGS in their guns will longer barrels. I’m no expert or even close so I have to rely on other people when it comes to these types of things. I’m just looking for recommendations so that I don’t waste money. So would you say that a +1 would be sufficient and/or maybe even shorten the barrel to 22” and a +1 gs
I also have rifle length gas on 24" barrels 224 Valkyrie, 308 win, 6.5 CM, that I can think of...most are 18".
Most all have adjustable gas blocks and they run fine. Many have been drilled out.
Most manufacturers use rifle length on 22 to 24 inch but some like to go plus 1 or plus 2 on long barrels.
It's not a big deal, just open up the gas port, or add just add adjustable gas block to fine tune...or both...depending on what port size the manufacturer uses, and your loads.
Some barrel manufacturers like to sell you their gas tubes so they make intermediate, or plus one.
I run a gaming 5.56 with a 16.5" barrel with rifle length gas system, no adjustable gas, speeds of 1085 rds per minute, have been attained with a binary trigger, with a titanium bolt carrier...no hammer follow through, and no malfunctions. It's a .122" gas port to function properly.
I've got a 22" with +2" and I wouldn't have it any other way.
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I'm shooting a 24 inch 6 ARC with a rifle length gas. Haven't had a problem and no brass damage.
FWIW the G mags cost an arm and a leg, but it's all I will run. Absolutely zero issues in 6ARC or 22ARC. I wish someone would reverse engineer the follower so I could put the Bambu to work 
I wonder if a 3d Scanner could get you close enough.FWIW the G mags cost an arm and a leg, but it's all I will run. Absolutely zero issues in 6ARC or 22ARC. I wish someone would reverse engineer the follower so I could put the Bambu to work
Are people having trouble getting 6 arc to feed? My ASC grendel mag has been 100% over about 1k now.
People don’t buy them because the mags are inexpensive. Beyond 10 rounds, ASC mags are the only Grendel mags I buy. Elanders and AA mags have been hot garbage for me. 10 rounders, on the other hand have been gtg regardless of manufacturer.
Well it only took a year but it’s complete. Front to back. Hart barrel, 24” 1-7 twist spiral fluted with RL+1 GS. Accu Tac bipod, Aero precision adj. gas block, advanced upper, lower and hand guard. JP LMBC, SCS and buffer tube. Rise Blitz defense trigger, rise upper and lower parts kits. Phase 5 extended bolt catch. Luth AR butt stock. Arken EPL-4 glass. Cerakote finish by Mycrow engraving and coating. All in all I’m pretty happy with the build. Down side is I had rotator cuff surgery before it was complete so haven’t shot it yet! Fingers crossed it performs as planned
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@Benhonda1 not sure you solved your problem but this is also for anyone else that is using these large case capacity rounds.
Just because your bolt doesn’t lock back after one round in the mag does Not automatically mean Open the gas port. You could have Too Much gas and very well may have a dwell time issue where your spent case is still expanded in the chamber while it’s trying to extract. This creates a malfunction that Looks Like it’s short stroking or low gas.
So it could actually be LESS gas or A heavier buffer or enhanced carrier similar to LMT will help give your expanded case more time to contract before it’s extracted. Or it could be a combo of less gas plus heavier buffer.
Just because your bolt doesn’t lock back after one round in the mag does Not automatically mean Open the gas port. You could have Too Much gas and very well may have a dwell time issue where your spent case is still expanded in the chamber while it’s trying to extract. This creates a malfunction that Looks Like it’s short stroking or low gas.
So it could actually be LESS gas or A heavier buffer or enhanced carrier similar to LMT will help give your expanded case more time to contract before it’s extracted. Or it could be a combo of less gas plus heavier buffer.
Check to see if gas port is fully in the center of the port on the gas block...with a bore scope, ..just in case it's not.
Too much gas, while the case is still expanded, the extraxtor, will bend or rip the rim right off the case head....easily noticed.
Tore up brass...
Too little gas will not function the bolt back all the way and/ or may not pick up the next cartridge in the mag.
Open up the gas port.
Too much gas, while the case is still expanded, the extraxtor, will bend or rip the rim right off the case head....easily noticed.
Tore up brass...
Too little gas will not function the bolt back all the way and/ or may not pick up the next cartridge in the mag.
Open up the gas port.
“the extraxtor, will bend or rip the rim right off the case head....easily noticed.“
While this is a sign, it’s not very common and it does not automatically eliminate Overgassed/dwell timing issues.
Just because the rim is Not getting ripped off or brass is Not getting damaged is entirely dependent on extractor spring strength and where the dwell timing is in the extraction process. It can still be over gassed and just Act like it under gassed.
This was a very common issue when 10.5” barrels first started coming out which brought about pigtail gas tubes. The balance between Gas port sizes buffer/spring weights weren’t quite dialed in yet.
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