Comparison Test; Pressure Trace II to Oehler M43 PBL

October 19, 2020

 I conducted an extensive test comparing the pressure measurements of the Pressure Trace II (PT II) as compared to the Oehler m43 PBL. The test rifle was a M1909 Argentine M98 Mauser actioned rifle with a 24” 308W chambered barrel on it. The barrel twist is 1 in 10” and the chamber was cut to minimum SAAMI specifications with a match reamer. Two strain gauges were permanently affixed to the barrel over the chamber 180 degrees apart at the SAAMI specified location for a piezo-transducer gauge. Thus giving, hopefully, the ability to record the pressure data from each shot on both systems simultaneously with each shot.

One gauge was attached to the Oehler M43 and the other gauge was attached to the Pressure Trace II. Set up was a bit involved as two computers (laptops) were needed as the M43 runs on the Windows XP program and the PT II runs on Windows 10 program. The M43 connects direct from the M43 console to the laptop. The PT II blue tooths from the console to the laptop. Also the PT II does not have chronograph capability so an addition chronograph is need for velocity manual input into the program. Thus, to give a full spectrum of capability, I set up both sets of screens [muzzle and down range in front of the 100 yard target] for the M43.

The test was conducted at the Sara Park 100 yard rifle range. Temperature ran 75 – 85 degrees during the testing, Humidity was 20% with a barometric pressure of 29.92. There was little to no wind.

Calibration with “reference ammunition” and use of the correction factor (CF). In past conversations with Dr. Oehler he suggested using a percentage CF instead of the differential CF that SAAMI uses. The differential CF used by SAAMI is only useful in a narrow range of psi measurements for any test barrel whereas the percentage CF is useful over a wider spectrum of psi measurements. The measured psi is multiplied by .95 to get the probable psi.

Unfortunately, the PT II system uses the SAAMI differential CF method. The CF is easily changed in the PT II program, but the CF is not consistent given different levels of measured psi. For example, with the four full power 308W loads tested a CF of 12,000 psi was very close. With a subsequent test of low end cast bullet loads in the 11,000 to 20,000 psi range a CF of 2,500 to 3,000 psi would have been correct. In the 20,000 to 30,000 psi range a CF of 3,000 to 4,500 would have been correct. Using the 12,000 CF which was correct with the full power load would have doubled the psi of the 5 gr Bullseye load under a 150 gr cast bullet. As it was, 8 gr Bullseye under the 150 cast bullet ran 21,200 psi with the M43 and 17,758 psi with the PT II both without a CF correction.

I used four different .308W loads of known pressure +/-.

Two Federal factory loads were used as reference ammunition. I only had a few rounds of each (4 and 6) as this ammunition was used to initially “calibrate” the barrel and to check its measurement after some use. A factory technician had given me the factory psi measurement of those lots of ammunition. Thus, when test fired in the test rifle using the M43 a “correction factor” (“CF”) was easily computed. The CF for the test rifle is .95% and has remained consistent, given other variables, throughout the barrels use. Someone may mistakenly think this is not valid reference ammunition but if one reads the SAAMI manual, they will find the use of a lot of factory ammunition of known psi is valid if standard SAAMI reference ammunition is not available.

The first Federal factory reference ammunition I used was the Federal 308W Premium loaded with Sierra 165 gr SPBT Game King bullets. The cases were FC, the primers Federal 210 and they contained 47.3 gr of a ball powder. Factory measured psi was 59,200.

The second Federal factory reference ammunition I used was the Federal 308W Standard load with the 165 Sierra SPBT Game King bullet. The cases were FC, the primers Federal 210s and they contained 42.0 gr of a ball powder. Factory measured psi was 52, 100.

A Lyman [50th Edition Reloading Handbook] shows a maximum psi tested load using the 175 Sierra MK over 45.2 gr of Varget in Remington R-P case, with Remington 9 ½ primers with an OAL of 2.800” giving 59,300 psi. I was able to exactly duplicate that load. That load, in last week’s first test ran 62, 300 psi [M43] (CF) and there was some “hard” bolt lift. The Lyman velocity listed (24” test barrel) is 2708 fps and the M43 recorded a muzzle velocity of 2754 fps from my test rifle 24” barrel. That 46 fps increase in velocity plus the harder bolt lift indicate that load with the components I am using is about 1 + gr over max. For this test I reduced the load to 44.0 gr of Varget with all other components remaining the same. We see, with this example, we still must consider the variation of lot to lot tolerance stacking +/- of the variation in components I used vs those lots Lyman used.

A “standard” M118 match load using the 175 MK in LC Match or R-P cases, Winchester WLR primers with 41.5 gr IMR 4895 with an OAL of 2.825” is stated to run at 59,000 to 61,000 psi. I also duplicated that load for testing. Again, as always when comparing our own loads to published loads we still must consider the lot to lot tolerance stacking +/- of the variations of components I used vs those lots Lyman used.

With the Federal 308W Premium factory load I only had 5 rounds left and one did not pick up on the PT II [operator error] so only 4 shots recorded on both systems. Not as many as I’d like but still gives us an idea. The PT II psi with a 12,000 CF ran 59,621 psi and the M43 after the CF calculation ran 58,900 psi. Given the factory psi was supposed to be 59,200 psi both systems were quite close as expecting “spot on” is an unrealistic expectation given all the variables involved.

With the Federal 308W 165 SPBT standard load I had 6 rounds left and all rounds recorded on both systems. The PT II ran 53,748 psi (CF) and the M43 ran 51,500 psi (CF) both of which, again, are quite close to the factory psi of 52,100 psi.

The Lyman Reloading Manual load was reduced 1.3 gr of Varget to a charge of 44.0 gr with all other components as specified. That load ran 59,542 psi (CF) with the PT II and 59,700 psi (CF) with the M43. Bolt lift and extraction were as to be expected with a top end load. The muzzle velocity was 2709 fps. That is, again, quite close to the Lyman manual psi of 59,300 at 2708 fps.

An excellent example of why we should always work up a load and how beneficial the chronograph can be in that endeavor.

The last full powered load tested was a standard M118 duplication load with the Sierra 175 Match King bullet. By the time I got to this load I was getting the hang of operating the PT II system consistently and in conjunction with the M43 System. Thus, I was able to get the 10 shot test string to record simultaneously on both systems with each shot. The PT II measured the psi at 61,064 psi with a CF of 14,000. A second 6 shot string with the CF set at 12,000 gave a psi of 59,464 psi. The M43 gave a psi of 59,800 with a muzzle velocity of 2645 fps. That is where both the psi and velocity should be.

Note the more complete data the M43 system provides. That is because the PT II system only measures the pressure. Any additional data is manually inputted such as load information (Note section) or velocities which must be gotten from a separate chronograph and inputted manually. The M43 PBL, as I have it set up, converts the average screened velocity to actual muzzle velocity The M43 can measure the time of flight to 100 yards or less and give the actual BC of each fired bullet. That measurement also is helpful in determining the stability of the bullet. There is also an external ballistic program to 500 yards which provides additional information. This is not to detract from the PT II but just is pointing out the differences.

The cost between the two systems is substantial and given the fact the M43 is no longer available adds to the attractiveness of the PT II system. Just be advised that if more than just psi is wanted then additional equipment and subsequent cost will be needed with the PT II system such as a chronograph and a laptop computer with Windows 10 on it that is blue tooth usable. Also, there is a learning curve with the PT II system just as there was with the M43 PBL system. Understanding what firearms the system can be used on and how the data is interpreted is essential along with a fundamental knowledge of internal ballistics. Neither system is for the casual reloader. One must be somewhat computer literate [more so than emailing and/or posting on forums] and have some ability working with equipment such as these as it can be frustrating to set up initially and to keep operational.

Additionally, some mechanical expertise is needed to properly glue the gauges on the test firearm. They are expensive and you will mess up one or two along the way. They also can detach or even break after use or miss handling.

I certainly am not trying to scare anyone off from getting a PT II or a M43, if one can be found, just pointing out it’s not a “run to the range and shoot” proposition. Planning for time and logistics of operation are essential as it’s not something you just run to the range on a whim to pop a few rounds with. If you have problems with setting up a chronograph at the range then neither system is probably going to suit you.

I am just beginning to learn about the PT II system and how to get valid information. Much more to learn for sure but I have several other comparative tests to run. For example, I think my next test with the 308W test rifle will be to attach both systems to the other strain gauge and test one of the same loads again. That should give an indication if the gauges are giving consistent measurements compared to each other.

Also, I am planning a test on my 44 magnum Contender test barrel. I will shoot a test (10 shots) of a known load with the M43 attached to the strain gauge. Then I will unsolder the wires of the M43 from the gauge and solder on the wires of the PT II and then shoot another 10 shot test of the same ammunition/load. That should also give a good indication if the PT II can be readily used on the other test barrels I have.

So much to learn, so little time……