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Old June 20, 2012, 04:32 PM   #1
ThisIsMySig
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Difference in CUP between two sources

Hey guys,

When comparing load data from two different manuals for a 125 grain 9mm LCN, I see the following:

My Lyman's second edition shows a CUP value of 27,600 from 3.2 grains of 231 with a velocity of 946 fps from an OAL of 1.115. The Hodgdon site indicates a CUP of 25,700 from 3.9 grains of 231 with a velocity of 1009 fps from an OAL of 1.125.

The CUP from Lyman's is higher with less of a charge than what the Hodgdon site indicates from a heavier charge. Is it possible that the difference in OAL is responsible for what appears to be a discrepancy?
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Old June 20, 2012, 04:49 PM   #2
Brian Pfleuger
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They use different guns, powder lots, primers, OAL, bullets, or bullet lots, cases, atmospheric conditions and test equipment.

Identical results would be more surprising than differences.
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Old June 20, 2012, 04:50 PM   #3
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Yes, it is entirely possible that the COL difference accounts for much of the pressure "discrepency." Other factors might be (1) bullet shape, which affects remaining powder space for a particular COL, (2) primer brand and lot number (3) powder lot number, (4) case brand, and perhaps even (5) some differences in the test barrels, although they are all supposed to be made to SAAMI minimum dimensions to maximize the pressure results.

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Old June 20, 2012, 04:52 PM   #4
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Quote:
Is it possible that the difference in OAL is responsible for what appears to be a discrepancy?
No way to know for absolutely sure, but probably NO!.

Possible differences include:
- different powder lots
- different primers
- different bullet length
- different testing equipment (how the pressure is measured)
- different firearm
- different atmospheric conditions
and so on and so on. It isn't a discrepancy, it is just a fact of load development and pressure generation.

The 9mm is sensitive to minor changes in seating depth (which is what really changes pressure, not OAL). But all the above reasons are why you always start at the beginning load and develop it for your firearm and components. And also why you should always check multiple sources before selecting your beginning point

Excellent question!
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Old June 20, 2012, 04:57 PM   #5
Jim Watson
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1900 CUP and 63 fps are peanuts even if in the opposite direction.
If you had the test gear you would be amazed at the variation round to round and box to box, even of The Same Stuff, never mind the book to book variables everybody has been talking about.
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Old June 20, 2012, 07:24 PM   #6
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Maybe measuring C.U.P. is not a repeatable process.
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Old June 20, 2012, 08:36 PM   #7
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Hey guys, thanks for all the responses.

Jim, I figured that the differences in velocity and CUP are not big, but I just found it odd that it was in the other direction.

Jepp2, doesn't seating depth affect COL and therefore pressure?

Brian, based on everyone's responses, I guess identical results across manuals would be more surprising.

Thanks again, great responses that really help.
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Old June 21, 2012, 09:07 AM   #8
Sport45
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Quote:
Originally Posted by Clark
Maybe measuring C.U.P. is not a repeatable process.
I think this is the best answer so far. It's not like they have a calibrated gauge on a test barrel that reads CUP. They measure the deformation of a copper pellet after the event. If the accuracy of the test was +/- 1000 CUP, I'd be surprised. They probably report an average for several firings and gain significant digits in the process...
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Old June 21, 2012, 03:51 PM   #9
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"They probably report an average for several firings and gain significant digits in the process...'

Absolutely correct. Strain gages and PSI is more consistant and even they aren't thought to be highly precise. There are some very large forces at work when a cartridge goes off and the total elapsed time is quite brief.
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Old June 21, 2012, 06:56 PM   #10
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Powder lots vary at least by 10%. The blending process is not held to a tighter standard.

Take a look at data in older manuals between W296/H110 and W231/HP -38.

These are the exact same powders sold under different vendor names.

You will see differences in pressures and velocities for the same charge weights, but you know, the powders are the same. What you are seeing is the lot to lot variation within the same powder.

These tiny differences allow gunwriters to drive Mac-Trucks down the middle. I have an article by Ken Warner on the 45 ACP and his load testing. In it, he deplores W231, says he "gave up" on the powder, and yet, HP-38 was one of the best powders in the 45 ACP he tested.

Now that we know they were the same powder, I have to wonder what he was observing. If he was seeing anything at all.
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Old June 22, 2012, 01:58 PM   #11
Clark
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My experience is that canister powders of different lots from different years are within 1%.

Bulk powders within 10%.

Load books with in 35%.
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Old June 22, 2012, 05:42 PM   #12
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Dittos for Mr. Clark!
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Old June 22, 2012, 06:10 PM   #13
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Different labs, different results. Anyone who taken a college course with a lab section is familiar with the this -- everybody is following the same procedure, but the results vary. The crusher test may not be an absolute, but it's results are relative to other tests done in the same lab.
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