Larry Gibson
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Completed my initial psi work up today in the 6.5×55 Swede with IMR4831 and the Hornady 129 gr SP.  Cases were Winchester and the primers were WLRs.  Charges were weighed on a Redding balance scale that has been checked for accuracy with verified weights.  The AOL of the loaded cartridge was 3.041″.  Test rifle was a Mexican small ring M98 Mauser that I installed a new, in the white, Swede M38 barrel.  The factory chamber was used as I was able to set headspace at minimum when fitting the barrel.  The barrel was shortened to 22.4″.

The strain gauge for the M43 Oehler was positioned over the chamber as per SAAMI location specifications.  Winchester Factory 140 gr SP ammunition was used as “reference ammunition”.  A factory technician gave me their pressure measurement for that lot of ammunition.  The psi measured in my test rifle was very close to that factory measurement.

After setting up the equipment at the range it was 49 degrees with 49% humidity and a barometric pressure of 29.71.  I shot 3 rounds of each increment from 42 gr to 50 gr in 1 gr increments.  That gives me a good idea of the psi range of those increments.  I have used this method with numerous cartridges the last 8 years and have found it reliable.  However, we must remember, as with velocity averages of different tests of the same load, so will pressure averages vary.  Not by a lot usually but the do vary.  Also different lots of the same powder will also give some variation in velocity and pressure.  As an example I shot the beginning load of 42 gr and then at the end of the test I shot a 5 shot test of 42 gr but of a different lot of IMR4831.  The main lot of IMR4831 tested ran 2220 fps at 36,400 psi(M43) while the other lot tested ran 2300 fps at 36,100 psi(M43).

Results of todays test of IMR4831 under the 129 gr Hornady SP;

Load       FPS       psi(M43)

42           2220         34,400

43           2300         37,500

44           2370         40,400

45           2460         43,400

46           2525         45,500

47           2590         45,800

48           2660         46,600

49           2750          51,300

50           2830          55,400

As noted in earlier post the SAAMI MAP for the Swede is 51,000 psi (transducer/gauge) and the European CIP PMAP is 55,000 psi (transducer/gauge).  A load of 49 – 50 gr puts us in the CIP PMAP range which was what I was looking for.  Now I will load two ten shot tests of 49 gr, 49.5 gr and 50 gr of my lot of IMR4831 and test them in my M38 Swede “scout”.   Given the 24.7″ barrel of the M38 I expect velocity to be 2800 – 2900 fps.

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