Observations

I’ve measured around 150 cases for CHExp including those in these tests. I have found measurements very consistent, particular when using new brass. I’ve tested in 35F, 60F, and 95F temperature conditions and can measure the resultant change. Based on the tests shown above, I believe max chamber pressure for the Accurate AA2200 data (SPC I and SPC II) and as measured with CHExp in new SSA Brass is occurring between 0.0040 and 0.0043”. Going forward, I am going to use 0.0045” as a maximum for load development for the SPC II, e.g., do not exceed, stop testing. I may learn more in the future that may change this but this is what I’m going to use for now. Also, if an accuracy node presents itself around 0.0040” it is a good time to take pause.

I used SSA because I had plenty of new cases. SSA brass also expands the most so is easier to measure and compare. Unfired, SSA brass typically measures between 0.4161 to 0.4162 inches at the base. Resized cases show much less expansion, about 1/3 of a new case depending on how many times it has been fired and resized. Note, once the quantity of charge is determined for a particular powder-bullet-COAL, the hand-loader can transition to using resized cases which will have slightly less pressure than new unfired brass.

You will see a couple in this first series with CHExp greater than 0.0045”. IMO, these are above max and are highlighted red in the results table. It is interesting to note that, once CHExp gets around 0.0045” and greater, the widest portion of the case is the bulging case wall forward of the base of the brass.

Other Observations

* Of the 3 bullets tested to the manufacture’s recommended maximum powder charge, all had slightly higher chamber pressure with the longer COAL than with the standard 2.260" COAL. One possible reason, being closer the lands can increases peak chamber pressure. 110gr Hornady, 120gr Hornady SST, and 130gr Nosler BT

* The 110gr TSX was the exception. Pressure was reduced with a longer COAL. Different from a typical bullet, the 85 and 110 TSX bullets don’t reach full-bore diameter at the end of the ogive. Full-bore diameter is at the first band (blue arrow in group picture). This means the 110 TSX has a lot of travel before it gets close to the lands. Also, the 110 TSX with its extra grooves has the same contact area with the bore as the 95 TTSX which is a benefit for internal ballistics for its weight. Note, even at 2.380” COAL, the 110 TSX is 0.050” off the lands which is Barnes’ recommendation to avoid high chamber pressure.

* It was interesting to note that the pressure dropped slightly when the 120 SST was loaded with additional powder and a longer COAL. I’ll have to investigate this some more when I focus on the load development for the SST with longer COALs.

* The 110gr AB load I was using was hotter than it should have been. Though I have fired at least a dozen rounds through my gun with this charge, it is above max and leaves no room for error, e.g., hot temperatures, dirty bore, etc.

* The 110gr TSX load has been running in my rifle for some time but the COAL has always been 2.295”. I should not have reduced the COAL to 2.260” for this test and resulted in higher chamber pressure.

2. Min to Max pressure curves with AA2200

Based on the test results above, I decided it would be interesting to measure CHExp with a ladder from min to max charge using the 110gr Hornady Vmax and two other lighter bullets in Accurrate’s load table, the 90gr Sierra HP and the 95 TTSX. Also, I needed to test CHExp for the 110 AB with a lighter powder charge.

18-inch ARP Barrel, 1:11.25 twist. The range temperature was 80F, about 10 degrees cooler than the first test. None of the Accurate loads were too hot. The graph gets a bit busier so take your time.

Observations

* The 110 AB loaded with 28.5 gr of AA2200 was within normal chamber pressure. It was interesting to see how the AB pressure spiked when maximum chamber pressure was reached, like running into a wall. This illustrates the need to have some margin. Be cautious if you think you want to run 100/110 Accubonds on the hot side.

* 110 Vmax and 110 TSX had a very predictable smooth pressure curves using AA2200

* 90 Sierra HP and 95 TTSX did show variations when reaching the maximum manufacture powder load with associated velocity drop-off/anomalies. Not sure why. More testing might provide some clues but I don’t have that many new cases left. I wonder if not all the powder is getting burnt after a certain point or if compression of the powder column is coming into play.

* Did you notice for the lead-core bullet velocities? Regardless of the bullet’s weight, the velocities basically matched for the same weight powder charge. They just had different chamber pressures.

* The 95 TTSX was shot last in this test series. My rifle does well with the all-copper bullets but I wanted to make sure there was no extra fouling that might affect test results.


Now that I am confident that high chamber pressures can be reliably determined, it’s time to get cracking and test some loads with longer COALs. First up will be the 110gr AB and TSX. You can already see there is a measurable benefit to loading long with the 110 TSX. I’ve already been to the range and collected data, just need a couple of days to finish the chart/report.

As shown previously, loading the 110 TSX longer resulted in lower chamber pressures. Below is the powder vs velocity data for two standard 6.8 powders, AA2200 and R10x, as well as three non-typical powders, 1200R, TAC, and CFE223. Velocities from an 18-inch ARP Barrel, 1:11.25 twist. My previously developed AA2200 load for a 2.295” COAL is listed with the star. All five powder loads with a 2.380” COAL achieved slightly faster velocities. Even with the longer COAL, AA2200 still achieved the highest velocities. The ladder done with CFE was done earlier this spring and I have misplaced the CHExp data. CFE has been typically running the lowest pressures around the 0.0030” to 0.0035” range. It doesn’t appear that the 110 TSX has enough internal friction to benefit from the slower burning powders like CFE.

Unlike the TSX, the 110 AB did not lower chamber pressure by loading long. Below is the powder vs velocity data for three standard 6.8 powders, AA2200, H335, and R10x, as well as two non-typical powders, 1200R and TAC (I’ll cover CFE223 later). The 28.0 – 28.5 grain AA2200 data shown is for a 2.295” COAL. None of the above five powders loads with a 2.375” COAL achieved faster velocities. Velocities from an 18-inch ARP Barrel, 1:11.25 twist. Note the velocity drop off with H335 as it reached its maximum charge. Caution: based on the AA2200 data, it appears chamber pressure with the 110 AB can spike when a max load is exceeded (unlike the TSX and Vmax which have progressive pressure curves).

I wasn’t planning on using CFE after seeing how it provided no velocity advantage with the TSX. However, when compared with the TSX, the AB had noticeably more internal pressure for the same powder charge. So I loaded up a ladder of 110 ABs with CFE and headed back out to the range. Positive results were achieved with a 2.375” COAL in all parameters; higher velocity, lower pressure, and better accuracy. Even with the different powder weights, all four bullets grouped within 0.75” at 100 yards. Note that a drop tube was used to get the heavier CFE powder charge into the case.

Here is a table and graph combining data from both bullets. It was interesting to see that the 110 TSX pretty much picked up where the 110 AB except for CFE which the 110 AB exceeded the 110 TSX. This was similar to how the different weight lead-core bullets did with AA2200 in the first chart.

You will also notice that the 1200R data was removed from the graph. My experience with this powder has not been good so far using 110gr to 130gr bullets. I had read in other locations on the forum how 1200R was providing higher velocities than other powders with 120gr SSTs without signs of swipe and primer indications of pressure. I too did not get swipes or primer craters, however, I did experience stretched brass, velocity drop offs of 40-100+ fps, and significant case-head expansion. My recommendations if you use 1200R for the 6.8mm SPC is to use the same powder weights recommended for AA2200, maybe 0.5 less.

Velocities from an 18-inch ARP Barrel, 1:11.25 twist.

My head hurts.

Vince wrote:

My head hurts

Yep, I think I'm starting to learn about internal ballistics. I was surprised to see that the same amount of powder yields close to the same velocity on so many different bullets - just the chamber pressure varied.

SlimJim,
Where are you measuring the case head expansion? I was taught that you measure the case head at the pressure ring - about where the flash hole is and that expansion beyond 0.0001 was over pressure. Since your measurement was up to 0.004" I assume you are measuring above the web?

PaulS

here are pictures of my micrometer and where I take the measurements. I basically place the micrometer's shaft at the base of the case, just above the rim. I found new cases for .243 and .270 expanded even more.

Ok, I see the difference. You are using an anvil mic and I was taught to use a knife edge mic. The larger anvil is actually measuring the expanded area of the brass above the pressure ring because it expands while the pressure ring doesn't go above the pressure ring at all. If you were using a knife edge mic about half way between arrow in your diagram and the base just in front of the grove your expansion would be difficult to measure with a 0.001" graduation.

I've never measured my case heads like this, but like PaulS says, what I have read was a blade mic that is accuarate to 1/10,000" should be used and head expansion of one thou is way past maximum (according to Hogdon).
www.hodgdonreloading.c...le-reloads

Victorian, thanks for posting. I'm actually measuring forward of the extractor groove on the case. Because of the width of the piston in my micrometer, I'm up near the thinner part of the base of the brass. I have found this very consistent and may be more accurate than what was explained on the Hornady website because the case expansion in this location in 10x than at the very base.