January 7, 2015, 08:09 PM | #1 |
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223 COAL variance
I'am loading Prvi Partisan 69 gr bullets in LC cases. The cases were all trimed at 1.755. When seating bullets I get a variation of coal of up to .013. I measured a few bullets and get .003 diference in bullet length. Dies rock solid tight. What could be causing this? Usually my coal is variation is about .003
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January 7, 2015, 08:29 PM | #2 |
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The shape of their pointed ends are not exactly the same. Typically, that much spread's caused by more than one set of bullet forming dies making bullets used in that ammo lot. More common with arsenal service bullets than commercial ones.
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January 7, 2015, 08:32 PM | #3 |
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What bullet type?
If they are the Soft points it might be lead crush when seating!
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January 7, 2015, 08:43 PM | #4 |
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What bart said.
Typically, with quality bullets, you will get .005 differences in OAL or so. Lesser quality in upwards of .015 |
January 8, 2015, 06:01 AM | #5 |
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They are spire tip HPBT's. I was thinking it was the bullet shape variance causing the issue. I figured when I shot these I would group them in lots of COAL and she how they group.
Thanks for confirming my theory. I'am looking for cheap accurate bullets. Is there such a thing? I have pretty good sucess with Hornady and Sierra Last edited by Fox84; January 8, 2015 at 06:07 AM. |
January 8, 2015, 08:47 AM | #6 |
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How many times have you reloaded those cases?
If the necks get old, they sometimes don't maintain the same neck tension and the bullets slide deeper into the necks as you seat them causing a shorter OAL than you thought you would get. Of course, inconsistent neck tension will yield inconsistent pressures and result in loss of accuracy. You didn't say if you also measured base to ogive on those rounds that were way off OAL. It the base to ogive measures are consistent, the problem is the bullets. If the base to ogive measures are inconsistent, then I would start to worry about the neck tension or even irregular pressure on the loading bar when you're working the press. Although I have never experienced your range of variations even when I was really aggressive with the press loading bar. It could be a combination of all of these potential issues. |
January 8, 2015, 09:04 AM | #7 |
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Fox84,
Few people realize that the distance from the case head to some part of the bullet is not what determines how far a bullet has to move before it touches the rifling. The case head is not against the bolt face when the round fires. All rimless bottleneck cases fire when their shoulder's hard against the chamber shoulder. Therefore, the case head will be some distance away from the bolt face. But it's not always the same amount. Here's two reasons why. Depending on how hard the firing pin smacks the primer, the case shoulder angle and the friction between case and chamber shoulders, the case shoulder gets set back a thousandth inch or more. That further puts the bullet closer to the lands. There's a small spread in how much this is for a given lot of ammo. Case headspace, the distance from the case head to the case shoulder that contacts the chamber shoulder is not the same for all rounds of ammo. There's a few thousandths spread. Cases with a greater dimension here will have their case heads closer to the bolt face when the firing pin hits their primer. Those with the least dimension will be the furthest away from the bolt face when this happens. Whatever this spread is, it causes the bullet to be positioned differently in the chamber throat. It adds or subtracts from the COAL's positioning of the bullet in the chamber. If two .223 Rem rounds have identically shaped bullets seated to a COAL of exactly 2.260" and one has a case headspace of 1.466" (SAAMI maximum) and the other's is 1.459" (SAAMI minimum), that .007" difference gets transferred to the bullet's position in the chamber. Which one puts the bullet closest to the rifling when its fired? And by how much? Rimfire5, I've seated a lot of bullets in cases with a wide range of neck tension on them. Never measured any difference between those seated in tight case necks compared to loose ones other than the normal couple thousandths variances across bullet's dimensions. It's my my experience that caseneck grip variables has no effect on bullet seating depth. I don't understand your reasoning that if the base to ogive measures is not consistent, the problem is the bullets. If you're referring to the base of the bullet, that doesn't effect what controls where the bullet seats. Incrementally heavier bullets with the same base and ogive shape but different body lengths have their ogives and tips seated to the same place for a given seating die setting. Their bases are incrementally deeper in the case by the same amounts. The small spread in base to ogive reference dimension for a given bullet determines how deep their base is in the case neck; the front part is the same across all relative to the case head. It's the shellholder face where the case head rests to bullet seater stem contact that determines the COAL length. Doesn't matter where the base of the bullet is. Last edited by Bart B.; January 8, 2015 at 09:22 AM. |
January 8, 2015, 08:19 PM | #8 |
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Bart,
When I referred to base to ogive measure I was referring to the cartridge base not the bullet base. That would give a comparison to use with the measured OAL which is measured from the cartridge base to the tip of the bullet. I never realized that soneone might think I was talking about the bullet base. I should have been more specific. Since I take the cartridge measurement all the time, I presumed that everyone did the same. The cartridge base to ogive is what you measure with the Hornady comparator gage using their modified cartridge cases. You could use the Hornady tool to measure the base to ogive of the bullet but that would only give you some idea of the variations in the bullet from manufacturing, which as you point out, doesn't really matter in comparing OAL to cartridge base to ogive. |
January 8, 2015, 08:52 PM | #9 |
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Rimfire,
OK, case head to bullet ogive, or cartridge base to bullet ogive in your words. But that will still vary when the rounds are chambered; their forward movement is stopped by the case shoulder and its measurements from the case head works with or against the head to ogive dimension. |
January 9, 2015, 07:03 AM | #10 |
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You're right about the position once the round is chambered, but the question was related to the overall length measured outside the chamber. That is really the only thing that the OP can control.
The case base to ogive measure versus the COAL measure variation would give some indication whether the OP's problem was due to measuring technique or actual variation in the bullets. Since bullet base to ogive variation is usually far less than bullet tip variations due to swaging, the case base to ogive measure variation should be much more stable than COAL. Last edited by Rimfire5; January 9, 2015 at 07:03 AM. Reason: corrected grammer |
January 9, 2015, 08:17 AM | #11 |
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One can control the case head to shouilder reference, too. That's done by where the sizing die's set in the press and the shell holder used. With Redding competition shellholders and the die set to stop hard against one of the right height so fired case shoulders are set back .003", case headspace can be controlled to less than .001" spread.
If one wants the distance bullets jump to the rifling to remain constant across the barrel's accuracy life, is he prepared to increase the case head to seated bullet reference point .001" every few dozen shots? Can they make that accurate of a change in their bullet seating adjustment? The rifling leade erodes away a tiny bit for every round fired. And the point on the bullet's ogive that touches the rifling is at a diameter a thousandth or more greater than bore diameter. Is the gauging point on your measuring tool about .221" for a .223 Rem's barrel with a SAAMI spec bore diameter of .219"? Any gauging point on the bullet .001" smaller than bore diameter may never touch the rifling; depends on how much bullet upset takes place when they're fired. Last edited by Bart B.; January 9, 2015 at 08:41 AM. |
January 9, 2015, 08:18 AM | #12 |
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Are you crimping at the same time your seating the bullet? Sometimes this affects OAL
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January 9, 2015, 12:05 PM | #13 |
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As Bart said, most likely this is due to different forming die sets that each wound up missing the exact bullet ogive radius specification after polishing and all else was done, but by a different amount. Most folks don't appreciate how hard that is to make exact. The result is the seating die, which usually pushes on the ogive somewhere, does not contact the bullet ogive the exact same distance from the bullet base for bullets off different dies, so it winds up seating them to different COL's.
The solution, if you have a lot of these bullets, is to use a bullet comparator on a caliper to sort the bullets based on the difference in measurement you get from the bullet base to the seating die's ogive contact point. If your comparator touches the bullet ogive at the same place as the seating die does, that will be most exact way to sort bullets to produce a matching COL. But that's not actually what you want for best accuracy. As Bart suggested, you want the shoulder-to-ogive measurements as close as possible for that. An not only that, you want the shoulder to the portion of the ogive that actually makes contact with the lands in the throat to be what you control. This can be done. Get a bullet and case comparator head for your caliper. I like the Sinclair for this because it is centered and not offset for a gauge like the Hornady is. But if you already have the Hornady, you can get their anvil adapter for the opposing caliper jaw to give you an opposing flat for the bullet bases and case heads. To use this approach, sort the bullets by their distance from base to ogive with the bullet comparator insert. Then sort your brass with the case comparator insert. Match the bullets to the brass. The longer the case head-to-shoulder, the shorter the bullet base-to-ogive you want by the same amount. This will give you a pairing that tends to produce cartridges that have the same distance between the bullet ogive contact point and the case shoulder, and that will tend to place the bullet the same distance from the lands during actual firing. You can argue about the necessity of going to all that trouble. Berger has found bullet seating depth sweet spots tend to be about 0.030" wide, and if you are seating down a fair distance from the lands, it's probably not important to get every cartridge exactly the same. On the other hand, you often see people who don't do the above, but who say they are seating just 0.010" off the lands, or sometimes even 0.005" off the lands. You can pretty well guess most of them have at least 0.005" variation after allowing for bullet and case variation, not to mention the firing pin impact setting the shoulder back a little. Sometimes by more than the 0.005" off the lands a shooter thinks he has.
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