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+--- Thread: Concentricity (/showthread.php?tid=223)
Concentricity - Phantom30 - 03-06-2021
I have found that long protruding bullets are vulnerable to several kinds of accuracy issues. It is well know that gas gun chambering eats the tips on soft lead bullets and can cause concentricity imbalance. I have tested it with 6mm Creedmoors using LR308 frames. The result at 100 yards with an standard hunting SP bullet was 7+ MOA, BTHP less than 1 MOA. Well the same thing applies with 6mm ARC beyond bullet tip problems. The gas gun barrel extension can also be a cause of flyers in a good group. I have two 20" barrels, Faxon and CMMG. Faxon has standard untouched M4 barrel extension while the CMMG is modified. Feed ramp contact during chambering can screw up seating alignment just as easily as poor hand loading.
Attached images show ARs and barrel extension scaring. Anybody else found this kind of issue?[attachment=173][attachment=174]
RE: Concentricity - grayfox - 03-07-2021
Since these pix all seem to be 6 Creedmoor, would be interesting to see some of actual 6mm ARC's with the same comparison. Also would be nice to see the comparison with same or similar numbers of rounds and types of bullets through each, so it will be apples to apples.
RE: Concentricity - Lemonaid - 03-07-2021
My opinion is that the feed ramp edges or corners mostly scratch the brass, at the neck and or the shoulder. A de-bur/polish of the sharp spots can fix the problem.
Chamber a new made dummy round or if safety permits a live round and eject w/o firing and inspect and evaluate. parallel marks about the width of the feed ramps are likely sharp corners that need work.
If the bullet is free from damage I would think case scaring would not make your groups much bigger, but you want to fix it to prolong case life.
RE: Concentricity - Phantom30 - 05-08-2021
(03-07-2021, 01:57 AM)grayfox Wrote: You should be able to delete the extra, duplicated pictures.These are all 6mm arc the only Creedmoor entry was the inclusion of the cartridge image to show the same 6mm PC bullet used by both
Since these pix all seem to be 6 Creedmoor, would be interesting to see some of actual 6mm ARC's with the same comparison. Also would be nice to see the comparison with same or similar numbers of rounds and types of bullets through each, so it will be apples to apples.
RE: Concentricity - Phantom30 - 05-08-2021
(03-07-2021, 03:09 AM)Lemonaid Wrote: My opinion is that the feed ramp edges or corners mostly scratch the brass, at the neck and or the shoulder. A de-bur/polish of the sharp spots can fix the problem.What I am talking about is the leverage induced on the seating alignment when chambering. The tip of the bullet contacts the barrel extension while the base is still held by the magazine. This torque effect can slightly change the alignment creating inaccuracy. My powder coated bullets are more effected than standard FMJ. They are seated at the base of the neck and with a gas check on the bullet that part becomes a pivot point for changing alignment versus the softer PC on the rest of the drive band. The other effect is impact for a soft bullet tip. Clearly using an SP round will open the MOA because of imbalance cause by denting the tip on chambering impact. Both are driven by the barrel extension that's why the comparison. I would shoot more to make a more consistent comparison but these effects are also exhibited in 6mm and 6.5mm Creedmoors. All three are to expensive to waste anymore.
Chamber a new made dummy round or if safety permits a live round and eject w/o firing and inspect and evaluate. parallel marks about the width of the feed ramps are likely sharp corners that need work.
If the bullet is free from damage I would think case scaring would not make your groups much bigger, but you want to fix it to prolong case life.
RE: Concentricity - Lemonaid - 05-09-2021
If you have an extra magazine that you don't care a lot if it gets ruined (or perhaps just the mag follower) you can experiment with the angle cartridges feed out of it. If it works make it a dedicated cast bullet mag.
I did that for a 6PPC Rap mag, made it so I could close the bolt on an empty mag.
A different brand of Mag. might work better (or worse!) so it's worth trying.
RE: Concentricity - BluntForceTrauma - 05-09-2021
I agree that this is an under-explored problem. Or maybe it's well-documented in the AR competition world?
I've seen 6.5 Grendel Wolf 123gr soft-point hunting rounds beat all to hell upon chambering. I'd think it would have some effect upon accuracy. (In the case of Wolf, the accuracy issues caused by smashed and bent and nicked tips was probably lost in the noise of the overall inaccuracy of the round.)
I've also seen plenty of scratches on HPBT bullets, but it's difficult to say whether they were scratched upon chambering or upon extracting an unfired round. Never did try it with the ejector uninstalled to eliminate the forces imparted by that variable.
Have also seen one of Hornady's new formula red plastic tips (more brittle?) in 6mmARC actually break off upon chambering and extracting an unfired round, so there's some force being applied there, as well.
Anyway, it's a problem worth solving, if solvable. My guess is we start by looking at how the magazine feed lips control the entry of the cartridge into the chamber, and then look at the geometry and finish of the barrel extension lugs.
RE: Concentricity - Dino11 - 05-09-2021
A few months ago I replied to a post on this forum about the same issue another member was having with 100 gr. SPBT Speer bullets.
I was having the same issue as he was with the point deforming when loading in the chamber. I carefully bent the feed lips so the tip of the bullet did not ride the feed ramp as hard but rather more on the side of the secant and the olgive. This was a tedious process as it is one of those cases that a little goes a long way, and no two brands of bullets are exactly alike. I have thousands of those 100 SPBT and my son loves to shoot them, and I'm glad he does because he would rather shoot steel targets than hone his skills on paper targets. They were cheap pulls I got, they shoot pretty damn good though. I use the messed up ones for barrel foulers after the barrel has been cleaned and de coppered.
RE: Concentricity - grayfox - 05-09-2021
Point deformation is one thing. I too have seen this in some of my soft-tip bullets and have decided to use them in other rifles because of that. But as to actually deflecting the entire bullet out of a radially concentric alignment due to forces on the cartridge while chambering, I would grant that there is at least a possibility of force, but unless we have some objective chamberings/re-pulls of various rounds with various lengths, seating depths etc, and measurements of resulting excessive run-outs... I'm reluctant to come to that conclusion.
Perhaps the advice of seating to at least 1 caliber's length into the case neck, plus adequate tension from re-sizing, offers enough resistance such that misaligning the bullet does not happen -- the misaligning force may be less than the force needed to offset the bullet. And the round could even be "banged" around while chambering, I'd even grant that. But don't think I've seen/heard much discussion on any great amounts of knocking bullets out of alignment... and I think that if it is a great problem, a) it would be generic to almost all ARS, and b) we'd be seeing a lot of discussion on forums about it.
So on that part, I'm still a bit skeptical.
RE: Concentricity - BluntForceTrauma - 05-12-2021
Grayfox makes some good points. Come to think of it, there are quite a few magazine-fed weapons in high-power competition, and they all have feed ramps....
RE: Concentricity - Phantom30 - 06-02-2021
(05-12-2021, 02:33 AM)BluntForceTrauma Wrote: Grayfox makes some good points. Come to think of it, there are quite a few magazine-fed weapons in high-power competition, and they all have feed ramps....Yes, but consider this in the AR community the majority of experience is with the 5.56x45 or 7.62x51 Either round is an FMJ class where the ratio of length to neck diameter as well as seating is relatively short. 6mm ARC is way longer and the casing neck is short. The magazine lips are long so the casing has a constraining pivot while the tip is engaging the feed ramp and counter bore.
Also most discussions about the problem deal with a jacketed bullet. I have been working with powder coated cast bullets. The lead will obturate at a mere 7 to 15kpsi, much less than 60kpsi for copper, plus the PC is also deform-able. So any impacts and you have the same concentricity issue you get with a SP tip only worse. As ammunition becomes more scarce it would nice to have a reliable accurate round you can make at home, given primers and casings. This is the reason for my concern with these issues
RE: Concentricity - CZ527 Guy - 06-03-2021
So am I to take away that the primary actions the AR shooter may do to maintain ammo accuracy in these semi auto loaders will be plastic tip bullets (smoother feed) more neck tension and a little finesse with achieving best feed from the magazine?
Are some magazines are set up better than others to insure a more smooth mag feed alignment?
RE: Concentricity - grayfox - 06-04-2021
What I take away is, my shooter induced errors are a lot more than any “mag-feed” bullet tip issues. When I can do an AR to 1/2 moa honest, then maybe I would look into other errors.
My 2 cents.
RE: Concentricity - Phantom30 - 06-06-2021
Well my CMMG shoots under a dime when I am having a good day with Hornady 105gr. The best bullet tip is more likely a reload using Hornady A-tip 110gr. Working on that.
I just replaced the Faxon barrel with a 22" Shaw. The barrel extension looks even better configured than the CMMG so we'll see. Half MOA would be good especially because I don't use a bench rig, just the bi-pod and little stock bag for stability. When you are as old as our President Donald J Trump, then heart beat control is additional fun. Oh yes i also have to endure the Monstrum G3 FFP with MOA reticle. Reticle has a dot not a cross hair on center which is a problem but it was affordable like all things related to Chinese corruption.
RE: Concentricity - BluntForceTrauma - 06-07-2021
Ran across an article about Service Rifle competition and, interestingly, the author was very much convinced that match bullet loadings can be damaged on their way from the magazine to the chamber. You can read the article HERE, but I'm also quoting at length below:
"The loading/chambering cycle of the AR-15 is, in a word, violent. You have a relatively heavy bolt and carrier assembly under power of a stout buffer spring slamming forward to chamber each round of ammunition. By design, those rounds were supposed to be held securely by the magazine feed lips until the bolt stripped them off the follower. Obviously, due to their overall length, 600-yard loads can’t be chambered that way. Although the barrel extension is funnel-shaped, it’s also got a lot of “teeth” ringing the inside of it.
"Whether you’re using a standard magazine or a single-round loading device (SLED), dropping a round in the loading port and releasing the bolt is an act of faith in that you’re trusting the bullet to arrive in the chamber undamaged.
"Ask yourself, how many times have you seen someone drop a round on the follower, press the bolt release and watch as the bolt jammed on a cockeyed round? If that happens once every 100 tries, how many times did the bolt close on a scratched, dented, or misaligned bullet? Could that be the cause of the occasional errant shot?
"My technique is to drop each round on top of the SLED and then push it slightly forward with my finger, partially chambering it before releasing the bolt. Admittedly, some people may be unable to do this due to body configuration or left-handedness, but why go to all the trouble of loading “perfect” ammunition and then damage it on the way into the chamber?"