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> Concentricity / runout
Concentricity / runout
@ 12:25 pm (GMT) |
Tom DixonI loaded a batch of Vartarg ammo last night, I checked concentricity of each round and sorted them adcordingly. The plan is to test the best against the worst and see what the real world difference is..When reading about concentricity it's not clear what the difference between concentricity and run out are, shooters tend to interchange the two terms but engineers see them as different things. The other thing I cave come across is some people refer to the total deviation whereas other half that figure and call it run out. I don't know wether the total deviation should be measured or the deviation from the central axis i.e half the total figure. My two worst rounds had a total deviation of .005... One thing to consider (and I'd appreciate some input on this) is the relationship between run out and concentricity. My understanding is this. If you had a perfectly circular plate and drilled a hole exactly in the middle the concentricity error would be zero. However if the drill bit was not at 90deg to the surface then a run out would be seen when a rod was placed in the hole and the plate rotated. So a concentricity error would occur when the brass was thicker on one side of the neck than the other, this would show up on the concentricity gauge and the error would be the same where ever on the bullet the gauge was placed. A run out error would occur when the bullet was not seated square in the neck, and would be larger when measured at the bullet tip and smaller when measured close to the case neck... have I got this right? |
Replies
@ 02:19 pm (GMT) |
Paul LevermanRe: Concentricity / runoutHi Tom - so, I'll start with the usual disclaimer: this is only an opinion and there are a lot of people here who know way more about this kind of thing than I do.My take on it is that they are two completely different measurements. Bullet run-out is used when magazine length (or a single shot rifle) allow the bullet to be seated within the optimum distance from the lands, using the case neck as the guidance system. Concentricity is used when magazine length prohibits the ideal COL, requiring a longer bullet jump and the associated guidance from the neck is no longer possible, so the round is loaded to as near perfectly concentric as possible, to assure alignment with the bore. I may be doing it wrong when I measure my rounds, but I use the extractor groove and a point somewhere between the the shoulder and the base of the neck, and I measure both fired (before sizing) and loaded (on the bearing surface of the bullet) rounds just for comparison. I have had problems previously with an inaccurate collet die, so now I'm paranoid. I believe this would only measure run-out. If you were to use the extractor groove and the bearing surface of the bullet as your contact points and take measurements at the case head, shoulder, neck, bullet, etc. you would probably see your concentricity errors. But, I am guessing. Somewhere in the reams of knowledge Nathan has given us, I think it was mentioned to avoid using the ogive due to manufacturing deformations. This one through me for a loop once as my indicator was just that much too far forward and the readings were erratic, to say the least. |
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@ 07:57 pm (GMT) |
Paul LevermanRe: Concentricity / runoutOoops - should read "threw me for a loop". |
@ 05:28 am (GMT) |
Warwick MarflittRe: Concentricity / runoutYes Tom you're correct. I can visualize what you are saying. You are bang on there mate. Concentricity is off center(not in the middle of the circle) and Runout is off axis.(not pointing straight ahead) easy eh! |
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@ 08:26 am (GMT) |
Tom DixonRe: Concentricity / runoutSo runout is measured as the deviation from the axis, I.e half the total measurement? |
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@ 02:03 am (GMT) |
Warwick MarflittRe: Concentricity / runoutDon't over thunk it! The point of the bullet needs to be pointing in line with the center of the barrel. It's that simple like a cork in a wine bottle. You don't put it in sideways..... As the arrows belowAre aligned so must the bullet and the bore ▶▶▶▶▶▶ →→→→→→→→→→→→→→→ Bang Bang I shot Bambi down. ..... |
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@ 03:13 am (GMT) |
Warwick MarflittRe: Concentricity / runoutYou can measure it any where the tip of the bullet will have the most wobble the best place to measure is it is in Nathans books. |
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@ 07:45 am (GMT) |
Tom DixonRe: Concentricity / runoutWell I went out to carry out my experiment, and discovered that my scope is fecked! I think it's the scope anyway, complete scattergun at 100 yds, two bullets touching then another 4" away, some missed the target altogether. So disappointing, I'm off to try today with a different scope.. |
@ 08:16 am (GMT) |
Mike DavisRe: Concentricity / runoutTom, while you have scope off, check the scope bases are tight then check the mounts aswell when you poke new scope on.loose bases will screw with your head...been there ,done that TWICE....... |
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@ 10:04 am (GMT) |
Tom DixonRe: Concentricity / runoutThey were tight unfortunately, it would have been nice if it was something so simple. I had a base mount screw shear the other day, really weird as they are m5 so not exactly light. I have never dropped the rifle so I assumed it was a manufacturing fault or maybe over tourqued. Anyway it must have had a knock of some sort as I suspect the internals of the scope have been damaged. I fitted a new screw and put the rifle away, yesterday was supposed to be a quick re-zero and off we go... Depressing times. |
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@ 03:12 pm (GMT) |
Tom DixonRe: Concentricity / runoutTurns out it's not the scope, there is a nick in the crown. I spoke to the smith who built it and he said that it's not uncommon for a wee flake of something to fall out of the moderator and be pushed out of the barrel by a bullet damaging the crown on exit. That's certainly what it looks like, it's pushed a tiny flake of steel off at the edge of the bore.. Oh well, re-crown and job done. And clean out the mod! |
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@ 01:19 pm (GMT) |
Tom DixonRe: Concentricity / runoutWell after getting it re-crowned I took it out to sight it in today, got it on paper with just a couple of shots, then tried a group, two shots touching then one 1.5" high and 1" right! Tried another, again two touching and a flyer. I got really depressed at this point, mentally going through a bedding job, followed by new scope etc. Then I noticed that the grass had grown a little since my last outing, crouching down and looking back at the muzzle I could see one or two stalks of grass waving in the breeze. On closer inspection I noticed a few stalks had been nipped off at about the height of the bullet path... Tried shooting off the bonnet of the landy and shot a nice group!.. ( .247MOA as it turned out) Idiot...rookie error.. Still I'd rather feel like a prick than have to start chopping my gun about.I tried the new Magnetospeed chrony too, works really well and so easy to set up. AV 3447fps, ES 26, SD 13..... So sighted an inch high it's point and shoot at crows out to 250yds. And only 4.5" low at 300yds. Concentricity? I've forgotten all about that now. |
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@ 12:17 am (GMT) |
Warwick MarflittRe: Concentricity / runoutWell how often has this been done? Grass Concentricity ? Good spotting Tom observing all conditions is part of the equation to eliminate possible causes. Enjoy your hunting. |
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