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nismochasse
09-22-2012, 19:49
I have a couple questions about some chrono results I got the other day. I was testing 155gr GDHP and 200gr BT hard cast, both over 800x. I shot two strings each of five rounds through a Pact chrony set up the recommended distance away (I think it was 8ft). Both were loaded to an OAL of 1.250.

Is it normal to get an extreme spread this large? What about the standard deviation?


155gr, 10.6 gr 800x, factory barrel
String 1/2:
Average: 1367/1366
High: 1407/1391
Low: 1348/1334
ES: 58.9/56.8
SD: 24.2/23.6
AD: 17.8/18.5

200gr, 8.6 gr 800x, 6" KKM barrel
String 1/2:
Average: 1271/1263
High: 1299/1280
Low: 1242/1244
ES: 56.4/36.1
SD: 20.0/15.4
AD: 12.2/12.1

Thanks for your thoughts.

Taterhead
09-22-2012, 22:08
That Beartooth load looks like a winner! That is about where I stopped with that bullet profile and 800-X, although you are getting more than 100 more fps than I get in my stock G20 barrel.

Spreads on the chrony are not abnormal. Not a big deal in a pistol. I would not at all be concerned as long as they group well.

TDC20
09-23-2012, 12:08
What you are seeing is very common. I have found this to be true with almost every powder and gun, rifle or pistol, at some charge level. And I always hand weigh charges for chrono testing to take that variable out of the data, so the charge weights are always within +/-0.05gr.

What I have also found is that most powders exhibit a "sweet spot" where for a given bullet weight (sometimes bullet type) primer,etc., there is a powder charge level where the ES goes down and the SD falls to single digits. Two pistol powders I've found trend heavily into that category are 800-X and Bullseye (don't use Bullseye for 10mm, of course!). Here is an example of my load workup (5 shot groups) with 180 XTPs at 1.255" OAL, 800-X, CCI#300, and virgin but sized Starline brass (new brass run through the sizing die is important for consistent brass tension on the bullet) and shot with my G20 and 6" LWD barrel:

9.0 gr. 800-X
Lo 1251
HI 1322
Avg 1286
Es 71
Sd 25.15

9.3 gr. 800-X
Lo 1265
HI 1324
Avg 1294.2
Es 59
Sd 25.72

9.5 gr. 800-X
Lo 1318
HI 1376
Avg 1339
Es 58
Sd 24.00

9.7 gr. 800-X
Lo 1342
HI 1376
Avg 1357.4
Es 34
Sd 17.11

10.0 gr. 800-X
Lo 1389
HI 1411
Avg 1400.8
Es 22
Sd 8.26

The same 10.0 gr 800-X load fired from the stock G20 barrel
Lo 1290
HI 1300
Avg 1295.6
Es 10
Sd 4.04

You can see how the Extreme spread and Standard deviation trend into that "sweet spot", and how that sweet spot carries over even to a different barrel.

I have read a lot of theories on why this occurs, and I have my own. Some say high % case fill is best for lowest velocity variation, and that seems to trend true for the most part, but not always. Of course, that requires that you select a powder with the correct burn rate and volume to match the cartridge capacity.

I've also seen good theories on bullet "stiction" interaction with the pressure curve (stiction is defined as variation in friction as the bullet traverses the barrel). There was one very good theory I read, probably the best I've ever seen according to soundly applied physics, about longitudinal barrel expansion and harmonics, which was actually written about rifle precision, but could also apply to handguns as well. There could be multiple effects at work, too. The level of barrel fouling definitely affects velocity, and that could also be related to the nature of the soot and deposits left in the barrel from the previous shot, which may be different for different powders.

There are a couple of caveats here that I would apply to Chrony data, too. I have used only the Pact Chrony over the years, and I will say unequivocally, that this instrument is not infallible. Sometimes, if the sunlight gets into the sensors or possibly the battery voltage is drooping toward the end of battery life, it will throw out an occasional bad velocity, or miss a shot altogether. So look at your strings, and if you see 4 very tight velocities, and one aberration, you might shoot that load again on a different day and find out that it is in fact a very tight velocity group. Also, 5 shots is a borderline "statistically significant" sample size, so you could probably shoot some groups repeatedly with very different results.

In any case, as Taterhead says, velocity spreads are not very important in pistol loads. Even in rifle loads, I have shot 5 shot .5" groups at 100yds with loads that spread 120fps or more, so it's not even that important in a rifle load, unless you're trying to make an 800yd shot, where 120fps might be the difference between a "10" and a miss.

Just for sake of how insignificant the difference is in bullet drop for your 155gr. load, I ran external ballistic data at both 1407fps, and 1334fps using Sierra's Infinity program. Here is the difference in bullet drop with both loads sighted in at 25 yards:

25 yds 0.0"
50 yds 0.13"
75 yds 0.41"
100 yds 0.82"
200 yds 3.71"
300 yds 8.44"

So, I would conclude that, unless you are an expert shooter with an extremely precise pistol, you would not be able to distinguish the difference in drop resulting from a 73fps (1407 - 1334) extreme spread in velocity at 100 yds. Even at 200 and 300 yards, to put this in perspective, a 3mph change in cross wind will affect your point of impact more than this amount of variation in drop (6" at 200 yds and 12.46" at 300 yds).

So why is it even a good idea to look at velocities and velocity variation? Well, first, you have a reference point of feedback to tell you whether you are "on" the book data you are using to develop the load. That means you know what your particular lot of powder, primers, bullets, etc. are doing in your gun. And second, if you dare to experimentally go where there are no book loads, as many here have, you can see when you have reached your velocity goal, and whether the load is becoming erratic as you approach pressure maximums, letting you know when it's time to quit with a particular powder/primer/loading. I have even seen where velocity reversals occur as the powder charge increases, which I consider to be a very dangerous thing. So there are very good reasons to use chrono data for developing loads, and I absolutely would not do load development without a chrono. But accurate, precise shooting loads, especially at pistol distances, are not necessarily a product of super consistent velocity.

nismochasse
09-23-2012, 18:36
Thanks TDC20 and Taterhead! You answered my questions and then some. Unfortunately, I didn't have a chrono for the loads working up to what I tested. I think I'll have to bump that up on my need to get list though.

Taterhead
09-23-2012, 19:15
Thanks TDC20 and Taterhead! You answered my questions and then some. Unfortunately, I didn't have a chrono for the loads working up to what I tested. I think I'll have to bump that up on my need to get list though.

Good call on that.

Good remarks by TDC20. I have a pet load for my heavy barrel .25-06 that I shoot long range (up to 7/8 mile- 1540 yards). We have wiggle room at my range for steel targets out to 2200 yards (1.25 miles). At those distances, velocity spreads are critical (as is BC and enough up travel in your scope), but closer they aren't so much. I use a charge of Reloader 25 under a Speer 120 gr BTSP that runs about 3100+/- fps depending upon temps. High-Low spreads for 5 shots are usually less than 20 fps. Some guys have loads that are tighter than that, but truthfully I am happy where I am.