Concentricity

Discussion in 'Long Range Hunting & Shooting' started by el matador, Sep 21, 2013.

  1. el matador

    el matador Well-Known Member

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    I've been learning a lot about this topic lately. I got ahold of a Sinclair Concentricity Tool and also a Hornady (which I will be returning). I have measured quite a few cartridges for neck runout and bullet runout. I hear a lot of people saying .002" or .003" and under is true enough for long range shooting. Is this measured at the bullet tip?

    Loads I have measured (270 WSM), indicator near the bullet tip.

    Reloads, Norma Brass w/Accubonds: .001-.003 with an occasional .004"
    Reloads, Win Brass w/SSTs: .002-.006 with an occasional .008"
    Factory Rem Core-Lokt: Most around .006 with some up to .013"
    Federal Premium 140 Accubond: .007-.009 with an occasional .013"
    Winchester Supreme Ballistic Silvertip: About half .001-.003 and the rest .006-.014"

    I sorted the Norma reloads and shot 2 groups: One group with little runout and one group with .004" runout. The straighter ammo shot 4 into .475" with one flyer out at 1.2", and the 004" stuff shot a pretty random group at 1.25".

    When you guys get nice tight groups with factory ammo are you sorting it by concentricity? Are heavy barrels less affected by bullet runout? .013" is a ridiculous amount and probably explains why my groups with factory ammo have never been all that great. I would love to hear your experiences with this.
     
  2. joe0121

    joe0121 Well-Known Member

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  3. MontanaRifleman

    MontanaRifleman Well-Known Member

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    I like to measure the bulletsshank/bearing surface if any is showing or down the ogive near the bearing surface. I have only received my Wilson concentricity gauge about a couple of years ago and it has helped me a lot in learning how to prep and size brass and seat bullets for concentric loads.

    That said, I have not seen any noticeable difference between rounds showing .001 runout give or take to loads in the .008-009 range. Not saying concentric loads aren't any better than non-concentric, I just haven't seen the diff. I have shot a lot of sub 1/2 MOA groups with runout in the .005-.009 range.

    A couple weeks ago I loded up 8 barrel accuracy test rounds for my new 6-284 before sending off for nitriding. 4 were in the .0005-.002 range and 4 were in the .005.-008 range. BTW, I had neck down the Lapua 6.4-284 brass to 6mm with a bushing die, nad that some times created neck runout. I had just cleaned the barrel so I thought I would shoot the 4 worse one first to sight in and foul the barrel. After shooting the first round, I adjusted the scope and shot the next three into a nice .4 group. This was just my mild break-in load and no accuracy development. I switched to a different target for the last 4 and the first of those landed in the same group area as the previous 3. The next 3 shifted left about and inch and printed about a .7 MOA verticla string for some reason making the total 4 group about 1 MOA for the "straighter" bullets. Maybe it was me? Moral of the story.... I think there are a lot of other factors that affect accuracy more than runout.

    Now, when you have everything else dialed in, maybe good concentricity will get you a little more precision on the target. I'm guessing it helps the BR shooters.
     
  4. MontanaRifleman

    MontanaRifleman Well-Known Member

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  5. Bodywerks

    Bodywerks Well-Known Member

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    In my opinion reducing runout to near zero is as important as ensuring consistent powder charges, neck tension, case capacity, and bullet bearing surface. There is an online article that pretty much says straight out that concentrically loaded rounds will result in tighter groups. I tend to agree, but it is but one of many factors that lead to tighter groups. Like I said, all other factors above need to be accounted for.
    Also, loading concentric rounds into a non-match, factory chambered barrel that may not be concentric is a wasted effort. This is easy to check. Use your concentricity gauge to measure your fired case runout at the neck. It should measure .001 or less, although ideally it will be practically immeasurable, with the needle only slightly moving off the powder residue on the neck.
    Also, like others have said, measure at the ogive or slightly forward of it. You are only concerned with the bearing surface that touches the rifling.
    The idea behind the importance of concentricity is two fold:
    1. It takes extra energy to force a non concentric bullet down the forcing cone and into the rifle. If powder burn energy is spending this energy it can affect muzzle velocity and make it inconsistent.
    2. The bullet may be forced into the rifling non concentric and the leave the barrel wobbling off axis which will result in less predictable bullet flight.
     
  6. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Interesting "theory"... the smallest group I ever shot was .224" @ 110 yds, It was factory ammo shot out of 25-06 Sendero. The runout measured in the .006 - .009 range.