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Old November 30, 2019, 02:36 PM   #1
tangolima
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QuickLoad help

Could you please run the following load on your software? The load on IMR's website just seems to have too high pressure for such light load. I will certainly follow their recommendation, but just want to have another point of reference.

Caliber: 7mm Mauser
Powder: IMR8208XBR
Bullet: Sierra #1950 RN 170gr 0.184"

IMR min load: 31.0gr, 2100fps, 39100CUP
IMR max load: 34.0gr, 2260fps, 45800CUP

Thanks in advance for your help.

-TL

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Old November 30, 2019, 04:24 PM   #2
Unclenick
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QuickLOAD doesn't do CUP, only psi. But if you look at the SAAMI MAPs in CUP and psi for this cartridge, they are 46,000 CUP and 51,000 psi. Multiplying the CUP by the ratio of the two, we get:

39100 CUP = 43,350 psi
45800 CUP = 50,778 psi

QuickLOAD gives the max load with 168 grain Sierra MatchKing 37,497 psi when I expand the case to get the same velocity Hodgdon did and I use their COL. But there are a number of reasons that can be off. We don't know how the lot of the powder QuickLOAD measured for the task compares to Hodgdon's average lot, so on. Actual as-fired case capacity and actual bore cross-sectional area are unknown to me. I would accept Hodgdon's measured data over the QL pressure prediction, for that reason, or use a Pressure Trace to measure it myself.
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Old November 30, 2019, 05:18 PM   #3
tangolima
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Thanks Unclenick. The result seems to confirm my gut feeling of the load being on the light side. But for sure I will start with the recommended min load, especially so considering my rifle is pretty old.

I think in this case something is amiss. 25% difference in pressure is way too much to be absorbed in normal component variations.

BTW, I read the relationship between cup and psi is close to, but not exactly, proportional. The equation I have is

Psi =1.52*cup - 17902

-TL

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Old November 30, 2019, 07:50 PM   #4
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That formula is for the least squares straight line fit to of a plot of Maximum Average {peak} Pressure (MAP) in CUP vs psi. It would be a good conversion recipe if the correlation coefficient were higher (very near to 1), but it isn't high enough, for my mone. For example, the .223 Remington and the 270 Winchester both are rated for 52,000 CUP MAP by SAAMI, but when they measured the same reference loads used to cross-calibrate copper crushers in the conformal pressure transducer, the .223 Rem came up 55,000 psi MAP, and the .270 Win came up 65,000 psi MAP. According to the formula, they should both be 61,138 psi. So it is 11% too high for the 223 Rem, and 6% too low for the 270 Win, and that's about as well as you can count on it to do. (See page 4 of this article where it originated.)

The load could still be on the light side. I just don't have an easy way to prove it without measurements.
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