May 28, 2011, 01:08 AM | #1 |
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Over all length math
My dummy rounds for my Hornady OAL gauge finally came after a long back order wait. I need help with the math.
1. The fired headspace for my Win 70 .308 is 1.625 2. The Hornady dummy round headspace is 1.621 3. So the dummy round is .004 shorter at the shoulder. I ran the bullet into the chamber and got the overall length at the ogive is 2.287, right up against the lands. So is the actual length to the lands (for my fired rounds): 2.287 + .004 = 2.291? Then do I start by deducting .020(?) and work backwards to find the "sweet spot"? Is there a maximum/limit to how far back to go before it gets too short. Thanks |
May 28, 2011, 05:53 AM | #2 |
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Are you talking about the Hornady Bullet Comparator that measures the length from the case head to the bullet's ogive? or Are you talking about the Hornady Headspace Comparator?
I don't understand what you mean that you got a dummy rounds for my Hornady OAL gauge? Usually you use the bullet comparator with a Gauge stick and a modified case. You screw the modified case onto the guage and insert one of you bullets into the modified case. You then stick those into you chamber and push the bullet to the lands. Then you meaure the ogive length. Do this several times to make certain that you have a good measurement. This is your maximum length. When you seat your reloaded bullet you reduce this length by 0.01 as a starting point. Try it. Then make the rounds a little longer, never going out to the max length. You never start at the max length and make them smaller. You start short and make them longer. |
May 28, 2011, 08:56 AM | #3 |
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Starting point depends on the rifle and caliber. Some rifles seem to like a little "jump" so I start @ .020" off the lands and work out slowly. If comparator length is 2.287" I'd start at 2.267". You're measuring the distance from the bolt face to where the bullet contacts the rifling, shoulder setback is a whole 'nuther can of worms.
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May 28, 2011, 09:08 AM | #4 |
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NOTE: this only applies to rimless cases. Rimmed cases set up on the rim & shoulder doesn't come into play.
You have the right idea with deducting the .004, but not the whole story. When you resize your brass you need to measure your headspace on fired & resized brass you make. then you need to figure out the difference (if any) between your h/s & the comparator's .004" undersize & use the answer to that as your start point for setting up the bullets. Personally I make several sets of 5 rounds at lands, lands-10, lands-20 & so on & fire for group.
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May 28, 2011, 09:28 AM | #5 |
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Hi Pa-Joe
This is what I meant that needs a dummy round: Hornady Lock-N-Load Overall Length Gage http://www.midwayusa.com/viewProduct...tNumber=570611 And I have the Hornady Lock-N-Load Bullet Comparator http://www.midwayusa.com/viewProduct...904&cm_vc=U013 And the Hornady Lock-N-Load Headspace Gage http://www.midwayusa.com/viewProduct...704&cm_vc=U013 Hi Wogpotter, Regarding: "When you resize your brass you need to measure your headspace on fired & resized brass you make" Does this apply to neck sizing only? Thanks guys |
May 28, 2011, 11:55 AM | #6 |
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"Then do I start by deducting .020(?) and work backwards to find the "sweet spot"?"
Since you know that seating against the lands is rarely good practice, it's also not needed, or even helpful, to know exactly where the lands start. It's sorta nice to get close but all we really need is a reference from which we will start load development. Agonising over 4 thou is meaningless. "Is there a maximum/limit to how far back to go before it gets too short." Well, if we overly compress the charge so the bullet push back up or they get so deep there's a danger they will fall in, it really doesn't much matter. Nothing that's in a rational way matters except for accuracy. Some guys get best accuracy with as much as .150" off the lands, some nearly on the lands, but seems most rifles prefer something in the range of 20 to 50 thou off. |
May 28, 2011, 01:31 PM | #7 |
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I'd check neck sizing once, just in case there was an issue. If it doesn't move then you're GTG IMO. All you're doing is verifying the shoulder is untouched that way.
While I agree that .004 is kind of borderline in relevance I'd definitely check several rounds for exact land contact distance. I'd use several because that would give both an average for the distance & allow for minor tolerance variations round-to-round, who knows maybe it'll be over .004 so you no longer need to worry about it? I don't know if the service is still available with the change in supplier but back when Stoney point made these you could send them a fired case from your rifle & they'd modify it to use as your measuring die case. You can also drill & tap a fired case to do the same thing but I don't remember the exact thread needed.
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May 28, 2011, 03:29 PM | #8 |
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Deepcore - You now need a Modified Case for the caliber you are reloading. The MC has a thread on the end to screw onto the Hornady Lock-N-Load Overall Length Gage. You put your bullet into the modified case and then measure the ogive with the Hornady Lock-N-Load Bullet Comparator.
wogpotter is correct that you also need to know your headspace since the case sits on the shoulder and the MC will not have the exact same shoulder as a fired round. But it will be close. You use the Hornady Lock-N-Load Headspace Gage (really a comparator not a gauge) to set your sizing dies so that you do not bump the shoulders back more than a 0.001". By minimizing how much you bounce the shoulders back you extend case life and keep the round more centric. |
May 28, 2011, 04:44 PM | #9 | |
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PA-Joe
Quote:
Help us out here deepcore do you have a case with a big thread where the primer usually is that screws to the end of the alloy "handle" of the tool?
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May 29, 2011, 12:16 AM | #10 |
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Yes, I do.
By "dummy round" I meant to say "Hornady Modified Case (in .308)". |
May 29, 2011, 07:57 AM | #11 |
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Something I've found to be really helpful with this tool is a 1/4" dowel.
What you do is push the dowel in the muzzles so you can jiggle bullets for exact lands contact. I found some rounds shapes made this a bit difficult to determine exactly but the dowel lets you "feel" the contact. Give it a try it worked for me.
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May 29, 2011, 10:37 AM | #12 |
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I've been using a dowel. One measurement run I did I found out that since the modified cartridge/brass has a loose neck to allow the bullet to slide in and out it can get a little off center in the bore, get stuck, and needed a little coaxing to push it back out. I think I pushed a liitle too hard with the tool on that try.
I took a peek down the barrel saw the bullet positioned tip up. |
May 29, 2011, 10:57 AM | #13 |
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You purchased/ordered/waited for delivery and received a dummy???? gage for a chamber you do not have, I make gages for chamber I have when finished I call the new creation a transfer/standard gage because? when I make a pile of gages, as many as 15 for one specific chamber as in 30/06, I avoid waking up in a new world every day and starting over, and I have more than one 30/06 etc..
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May 29, 2011, 01:24 PM | #14 |
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It does need a fairly gentle touch to work well.
I use most pressure on the sliding bullet pusher of the tool & just light (1 finger) resistance on the dowel.
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May 29, 2011, 01:25 PM | #15 | |
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Quote:
What thread is it in the back of the modified case? I forget.
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May 29, 2011, 03:11 PM | #16 |
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5/16-36 I believe.
Working on making one from my fired cases but working out keeping the hole I'm drilling centered. And brass is grabby so I needed more drills bits to slowly step up/enlarge the hole in increments ( til I get to the needed 9/32 bit size). Time for the big set of crayons so to speak. |
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