High altitude 10mm chrono readings, help?

[Strykervet]

I am working up 10mm loads. My data is from Hornady. The loads are 180gr. XTP ranging from 8-9.8gr. 800-X (the max is 10.1 at 1250fps). 155gr. XTP ranging from 11-12.4gr. Blue Dot (max 12.9 at 1450fps). 200gr. Hornady FMJ ranging from 8-8.4gr. Blue Dot (max 9.4 at 1150fps). All loads using CCI 300 primers and once fired Starline brass. All loads weighed and measured, COL is published numbers. Crimp is a little more than necessary for the Glock barrel (I test the crimp in a match barrel, just enough to fit). All shot from a basically stock G29 using a stock barrel and stock spring. All brass looks fine with no "glock smiles" and very, very minimal bulging, if any, and the primers look fine too. No visible signs of pressure.

Now the first three loads in 180gr. were fired at an army range not much above sea level at about 45deg. F. Using 800-X, I got: 8gr., 990fps, 8.5gr., 1050fps, 9.0gr., 1150fps.

Then I loaded 9.1-9.8gr. in 1/10th gr. increments and fired them at about 4000ft. in about 85deg. F. weather. It was the best place I could find off-hand. I got: 9.4-9.5gr., 1129fps, 9.6gr., 1130-1155fps, 9.7gr., 1157-1173fps, and 9.8gr., 1174fps.

At the high altitude, I also tested the 155gr. XTP's. I got 1081fps for 11gr. of Blue Dot, 1173 for 11.9gr., same for 21.1gr., finally 1220 for 12.4 gr. Using the 200gr. FMJ, I got 866 for 8gr., same for 8.2gr., and 875 for 8.4gr.

These numbers are way off from the book numbers (which was using a 5" bbl). However, the low altitude shot, 9gr. 800-X at 1150fps, that was dead on the money regarding the book data even with the short Glock bbl. But when I loaded up close to the max, when I got to the 9.8gr. loads, the highest I got was 1174fps!

But the biggest disappointment was with the 155gr. bullets. The highest reading was 1238fps using 12.4gr. Blue Dot, only .5gr. from the max. The book lists 1400. The 200gr. gave 897fps using 8.4gr. Blue Dot, with the book being at 1025fps for the same load.

To put all this in contrast, the G27 registered 1110fps using Winchester Ranger T-Series 165gr., advertised velocity is 1180fps. I had some 5.5gr. N340 and 180gr. Rainier .40 loads that seemed to be closer to published data than the Blue Dot and the 800-X, but not the Ranger loads.

So the questions, do you think it is the powder? These low pressure high volume powders at high altitudes? Because whatever Winchester uses in their RA40TA worked fine. Does anyone have experience with this? What happened when you worked the loads up again at lower altitude?

I really expected more out of these loads, I really did. I suppose all those loads will have to be worked up again, because if pressure is the culprit, well, I really don't need to say more. Anyone with any help or experience here would be appreciated, as I live in an area where I can be anywhere between sea level and 5600ft. I'd like a real 10mm load that will perform consistently well over a broader temperature and barometric range.

 

[steve4102]

How far from the muzzle was your Chrony set up at both locations?

 

[GooseGestapo]

I suspect the difference is with the chronograph itself.

Distance to screen from the muzzle affects the instrumental reading. Light angle could concieveably affect the readings as well.
Lower pressure more likely has more affect on the chrono than the ammunition. Although I've never seen such issues with aircraft electronics at higher altitudes or conditions otherwise well tolerated by humans. Extreame conditions (above 60,000ft and above 50degC or below -20deg C) can induce problems with eletrical equipment not sealed against pressure or vacumn.

Like you, I'm puzzled by the differences in the readings. Typically, higher temps mean higher readings, as does higher altitude with lower pressure yeilds increases in velocity as well as ballistic coefficent. Your results are the inverse of what I'd expect.

If all else is equal (components, chronograph, firearm) the chronograph is most likely to be the issue.
I've gotten some extreame fluctuations with chronographs that were chrono related such as battery condition, screen condition, light angle, ect....
Also, you mentioned two different firearms. A G29 and a G27, and you mentioned using Winchester .40S&W ammo ?????

 

[243winxb]

So the questions, do you think it is the powder?

Could be the powder. Looking at the Hodgdon website, the 180gr sierra - 800X would seem odd that they dont list a starting load. Very low pressure compared to the other powders. Different bullet, different results. The altitude should not effect the burning rate of the powder as its energy is self contained. The air density at long range would have an effect, but not to 15 feet to the chronograph. When setting up the chronograph at different locations, have a firearm & factory ammo that will produce known* result (fps) as a test sample. No experence, just a guess.

 

[gamestalker]

I've noticed with an etreme difference the increased velocity at high elevation. I lived at 7,500 ft. and shot my .270 win. using the same load I do now and experienced an increase of over 200 fps.. My load that now shoots 3150 fps at under 2000 ' was averaging a little over 3350 fps at 7,500'. Same chronograph, and same distance from the muzzle.

 

[Strykervet]

How far from the muzzle was your Chrony set up at both locations?

Between 10-12 feet on average.

 

[Strykervet]

I suspect the difference is with the chronograph itself.

Distance to screen from the muzzle affects the instrumental reading. Light angle could concieveably affect the readings as well.
Lower pressure more likely has more affect on the chrono than the ammunition. Although I've never seen such issues with aircraft electronics at higher altitudes or conditions otherwise well tolerated by humans. Extreame conditions (above 60,000ft and above 50degC or below -20deg C) can induce problems with eletrical equipment not sealed against pressure or vacumn.

Like you, I'm puzzled by the differences in the readings. Typically, higher temps mean higher readings, as does higher altitude with lower pressure yeilds increases in velocity as well as ballistic coefficent. Your results are the inverse of what I'd expect.

If all else is equal (components, chronograph, firearm) the chronograph is most likely to be the issue.
I've gotten some extreame fluctuations with chronographs that were chrono related such as battery condition, screen condition, light angle, ect....
Also, you mentioned two different firearms. A G29 and a G27, and you mentioned using Winchester .40S&W ammo ?????

Yes, I am puzzled. All components were the same. FWIW, the G29 ejected brass quite far, as would be expected with full power loads... I wondered if perhaps the recoil spring was too light and didn't keep it battery long enough.

The G27. I fired some practice rounds to test out. They performed as expected. Also, I fired some RA40TA Ranger loads, 165gr. T Series. Those chrono'd as expected as well --just a little off from the advertised rate, but considering the short barrel, they were well within what I expected.

The chrono. It is new and has performed well so far. The M4 rounds fired up there clocked in as expected too...

Only the 10mm was the issue, and yes, it was the inverse of what I expected.

The sun WAS shining at an angle on the chrono... Not much I could do about it. But it still doesn't explain why the others worked fine.

Altitude? Maybe, that I why I was wondering if anyone on here had similar results. On the other hand, it could be the recoil spring. These loads aren't "hot" but they should be more powerful than watered down commercial ammo. The other culprit could be the powder. The Blue Dot and 800-X both fill the case and burn slow. Perhaps the barrel is too short to fully take advantage of these powders. I won't be able to compare this against the G20 until Glock sends my frame back (ambi mag release design is crap --doesn't work reliably at all).

Still, keep the hypothesis coming. I want to get to the bottom of this. Right now, I blame the combination of altitude and slow burning powder in a short barrel. I haven't shot it in the dark yet so as to get a visual of just how much unburned powder is escaping as flash. I haven't tried any loads using 3N37 or N340 yet either, both of which may burn better. I don't suspect the chrono yet based on other readings.

 

[Strykervet]

I've noticed with an etreme difference the increased velocity at high elevation. I lived at 7,500 ft. and shot my .270 win. using the same load I do now and experienced an increase of over 200 fps.. My load that now shoots 3150 fps at under 2000 ' was averaging a little over 3350 fps at 7,500'. Same chronograph, and same distance from the muzzle.

Yeah, that makes sense. That was what I expected, and why I am leaning toward the performance of slow burning powder at altitude combined with the short barrel... Perhaps it is just the powder/barrel combo, but I have heard of similar loads generating much higher readings. I just don't get it yet, and like I said, I won't have my G20 up for a few weeks at best to compare...

Again, thanks for the insights, keep 'em coming!