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Old March 4, 2012, 09:15 PM   #1
Kayser
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Bullet length as it affects COL.

This is something that you pretty much never see discussed, but which has always seemed extremely relevant to me. In short:

- A given recipe might specify a specific bullet (eg, 240 gr hornady XTP), powder charge and OAL.
- Pressure is dictated by charge weight, bullet weight and OAL of the round (seating a bullet deeper shortens OAL, decreases internal case volume and increases pressure).
- Not all bullets of the same weight are the same length.

Therefore, it seems to me you must adjust OAL by the difference in bullet length from your reference bullet.

For example, my Hornady manual lists a COL of 1.600" for their 240 gr XTP 44 magnum bullet. Ok, great. But let's say I have a bag of Magtech 240 gr bullets. If you measure them, the XTP's have an average length of 0.712". But the Magtech's have an average length of 0.685"

It seems to me that this implies if you are using the Magtechs with the same powder charge, you must shorten oal by (0.712" - 0.685" = 0.027") to keep the internal volume and therefore pressure, the same.

This seems quite obvious and relevant, but you pretty much never see this discussed in reloading circles. The manuals don't list bullet length (at least the ones I've seen).

Anyone else find this weird?
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Old March 4, 2012, 09:28 PM   #2
jepp2
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What determines pressure is bullet seating depth. You math is on track. Determine the bullet seating depth of the load listed in the manual. If you use a bullet with a different length (but the same weight) adjust your OAL to provide the same bullet seating depth for equivalent pressure results.

I have an older Lyman manual that did list bullet seating depth for each load. But it is easy enough to determine and adjust. For bullets you don't have the length of, this link bullet length should prove useful.

It gets discussed all the time. I have seen at least 5 threads in the past week on seating depth here.
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Old March 4, 2012, 10:04 PM   #3
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Kayser,

You are spot on. I posted in another thread here a couple of examples of changing the COL, and the velocity changes slightly but the pressure changes quickly in increments of 0.010". This is why for dramatically different bullets such as Barnes all copper you should use data from the bullet manufacturer. IIRC most reloading manuals have a caution about using different bullets than the ones listed.

I always run my test load data through a program called QuickLoad that will calculate the changes based on myriad factors, including COL.

Before I had Quickload, if I used a different bullet than the one I could find listed, I would either slightly adjust the powder or the COL to compensate on my work-up loads.
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Old March 4, 2012, 10:25 PM   #4
Kayser
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I've just gotten in the habit of taking some measurements of every bullet type I buy and keeping them in my log book. Since I use the Hornady manual, I'll usually just pick up the smallest possible box of actual Hornady's as a reference point if I'm going to use something slightly different.
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Old March 5, 2012, 09:35 AM   #5
wncchester
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"Anyone else find this weird? "

I don't. When I started reloading few data sources bothered to 'suggest' any OAL at all. We were expected to intelligently find a length that worked in our own firearms and develop our charge. We did; we lived.

There are still a lot of bullets available that no book maker has told everyone exactly what OAL to use; if we can't figger out a seating depth that works for ourselves I suppose we should not use those bullets.
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Old March 5, 2012, 12:10 PM   #6
Kayser
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Well ok, but it seems to me one of the possible failure modes of trying to figure it out yourself is "when your gun blows up". It's one of the parameters that effects pressure, just like charge and bullet weight. Just like we would expect someone with proper experimental apparatus to figure those out as a starting point, it would seem like we should expect the same for oal.
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Old March 5, 2012, 01:29 PM   #7
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"Anyone else find this weird?"
Not at all. Manufacturers don't want you using their data for other folk's bullets (why buy several manuals when one will do?) and they don't want you to have any direct comparison between their data and someone else's.
As said, you can't always go by overall length. The Hornady 158gr .357" XTP-FN and the Sierra 158gr .357" JSP will give almost identical overall lengths (.003" difference) but the difference in seating depth is .012" (.360" for the Sierra and .372" for the Hornady). I don't have the numbers handy, but I believe the difference in seating depth between the Sierra .357" 158gr JSP and JHC is around .035"...enough that they can't (or at least shouldn't) use the same maximum loads.

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Old March 5, 2012, 01:43 PM   #8
.45 Vet
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I'm guilty of not using the online reference manuals very often. I get most of my data from hard bound books. Don't they still suggest that you start low and "WORK UP" your load when changing components from the listed data ?
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Old March 5, 2012, 01:52 PM   #9
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Yes, they do...including starting from scratch whenever you buy a new lot of powder. That being the case, their data is only a "best guess" as it wasn't created using exactly the same powder that you have.
If that sounds cynical, it is. I shoot the .357 Maximum in a Dan Wesson SuperMag and attempting to go thru published data to achieve an optimum load is an exercise in patience and futility.

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Old March 5, 2012, 09:04 PM   #10
wncchester
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"Just like we would expect someone with proper experimental apparatus to figure those out as a starting point, it would seem like we should expect the same for oal."

'Suppose maybe it does seem that way. But, THEY have THEIR guns and WE have OUR guns and they DO give us starting points which, being only starting points, are by no means deadly critical; no one has ever blown their heads off with starting loads so it's a mote point.

If we are uncomfortable setting our own starting seating depth we can just make it 'look' right - somewhere rationally between the bullet falling out or falling in - will be good enough for starting. After that, it's up to us no matter what any book suggests for powder charge or OAL; that's why they all tell us to "start low and only...", etc. and that admonition includes any pressure effects from seating.
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Old March 6, 2012, 11:54 PM   #11
DWFan
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I found the pic I was looking for. This shows seating depth difference very nicely. Same bullet weight, same profile, same length; only the crimp groove location and lube grooves are different. They cannot use the same load data. The bullet on the left will reach maximum pressures much sooner as the powder charge size increases.
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Old March 8, 2012, 12:04 AM   #12
black mamba
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Everything about different bullets (of the same weight) is different but the weight. Not just crimp groove position, but diameter (44 mag bullets come in .429, .4295, .430 in jacketed, and even larger in cast), ogive, bearing surface, jacket thickness and ductility, core hardness, etc.

The point is, these little differences don't amount to enough of a difference to adversely affect pressures, as long as you . . . .
"start 10% below max and work up, looking closely for pressure signs."

They could make a catastrophic difference if someone was careless enough to start at max and hope for the best.
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