I just got back from the shooting range and I am confused!!

Yesterday I developing a load for my 30-06. Norma cases, North Fork 165 gr bullet, Winchester mag rifle primers and Norma URP powder. Started out at 53 gr and shoot 3 rounds of each load increasing by 1 gr at the time stopping at 57 gr. Groups went from about 1.4" down to under 1/2" from lowest to highest powder charge. Velocity went from 2600 fps up to 2890 fps. The 55 gr load gave a shot to shot variation in velocity of about 100 fps, the other loads were within about 10-30 fps in variation. Cases extracted smoothly and looked good, no signs of pressure. So I was happy!



Today I decided to try adding one more gr of powder (58 gr) to see if the positive trend would continue. I also loaded up 5 rounds of the best load from yesterday (57 gr) to see if I could produce a nice 5 shot group to brag about and to boost my ego for upcoming hunts.


To my surprise the 58gr load gave velocities exactly like the 57gr load did yesterday, right at 2890 fps (extreme spread about 10 fps). 5 shot group at about 1,5". I got even more surprised when the 5 shot group of the 57 gr load averaged almost 100 fps slower than yesterday and with a good deal of extreme spread in velocities. Accuracy how ever was still excellent, 5 rounds into about 1/2".


Powder was thrown and measured on the same RCBS ChargeMaster for all the rounds. I am new to chronographs and testing handloads in this manner so I am looking for your experience.

Am I doing some thing wrong or is it normal to get this much variation in velocities of identically loaded rounds? (yes, same batch of all components). I used an old Shooting Crony to measure velocity.

Atmospheric conditions could have had an influence. Do you record atmospheric pressure, temperature and humidity?

I don't know how temperature sensitive Norma URP is but if it's like MRP or MRP II it should be pretty tolerant. That said, temperature can make a significant difference in some powders. I use IMR 7828 in a couple of cartridges and it's a bit temperature sensitive. An example would be the load development for my 338-378 was done when the temperature was about 68 degress F at about 3150 fps with 225 grain bullets. I shot this same load when it was 108 degrees F and they were running about 3220. I started working up a load for 265 grain bullets using US869 which is a very slow, temperature tolerant powder and the readings didn't vary more then about 20 fps with 3 identically charged cartridges. The same readings were observed again when the temperatures were about 20 degrees different with this powder.

Also understand that chronographs are also only so accurate and lighting can affect the readings. They are a handy tool to see what's going on with your loads but they are not the end all indicator. You want to make sure you are taking the readings about 15 feet from the muzzle so that the gases are not being read by the chrono.

Just my 2 cents.

Ken....

Temperature can have a significant effect on velocities. I generally expect a 100 fps change from freezing to 70 degrees f, and an additional 1-- fps change fm 70 to 100 degrees f.

You've just began a long process of understanding the subtleties of understanding what your rifle is really doing.

In general, an extreme spread under 30 is good. Under 10 is excellent.

I suspect that there may be more variation in the readings from your chronograph than actual variations in your loads.

The little Chrony is an economical and useful machine -- as far as it goes. Due to its design and short screen spacing it is a bit prone to irregular readings. A complicating factor is that it may register velocities slightly differently each time it is set up due to its "folding" screen arrangement. You see, if its hinge point and hinge stop is slightly impefect, then the sreens may be pointed slightly inward or outward compared to the previous time it was used. Tilting the screens slightly inward (leaning toward one another) results in an effectively shorter screen spacing, meaning higher indicated velocities. If the screens are tilted slightly outward (away from one another), then that results in lower indicated velocities. The further toward the top edge of the screen detection area (higher) the bullet passes, the greater the effective variation in screen spacing due to this tilt.

A chronograph with a longer (four or five foot) screen spacing and fixed screens will give you much more consistent readings from one shooting session to another.

Stoney, I've had single digit standard devisions with my little unreliable chrony. 5 shot stings varying less then 10 fps.

Welcome to the world of statistics. Anything less than about 20 rounds is not a meaningful sample. Before you go ballistic on me (pun fully intended), I typically do 5 shot strings myself but know that the findings are suspect. Whether you are talking accuracy, velocity or whatever, once you have a load that looks promising, you need to shoot a minimum of 10 rounds to know if you are really in the ballpark or just looking at a fluke.
If 10 rounds still looks promising, shoot 10 more.

Two things to assist with consistancy.

Set the Chrony up the exact same way every time.

My display cord has tape on it to measure exactly 15 feet.

I use the same stand at the range whenever Chronying.

Pay attention to the ambient light (with/witout diffusers).

Another is the condition of the inside of your case necks. Is there graphite residue remaining in the necks from re-sizing?

I alway clean the necks the same from each batch of brass and also use a Lee Factory Crimp Die. Really brought my ES way down.

quote:

Originally posted by Antelope Sniper:
Stoney, I've had single digit standard devisions with my little unreliable chrony. 5 shot stings varying less then 10 fps.

I'm sure you have. However that doesn't mean that the actual velocities were as close as the machine recorded them. You see, with only a 12" spacing and a 1MM cycle "clock", the closest reading you can get under perfect conditions is about plus or minus 8 fps -- which is not bad but means that 5 shot string with less than 10 fps of indicated variation is more likely simply an accident of rounding. That doesn't mean that the Chrony won't tell you the difference in a load that has an SD of 130 vs. one of 25. It will, within limitations.

I know that it's a popular thing to knock the little Chrony, and I'm not doing that. As I said, its a useful machine as far as its limitations go. And its internal works (clock, circuits, etc.) are essentially identical to the more expensive machines and the Chrony's internals operate practically flawlessly insofar as the inputs they receive.

The Chrony uses an inexpensive screen system which is a bit "hit and miss" as anyone who has used this machine can testify. Not only does one screen or the other sometimes fail to detect the bullet (which you can put up with for the price), but the screens also "read" the bullet in places where it is sometimes "not" exactly. Any screen system has some degree of error, but the Chrony's latitude of screen error (parallax, if you will), is some bit wider than some of the more expensive machines.

All of the above introduces some degree of error which is not present (to as great an extent) in more expensive machines.

But the largest potential source of error from one day to another is in the manner in which the screens are positioned, ie., by folding open 180 degrees at a hinge point. This is an ingenious design for purposes of making the machine compact, portable, and quick to set up, but it necessarily introduces yet another source of error: Screen spacing. Again, with the very short 12" screen spacing to begin with, by tilting the screens just a bit inward or outward compared to the "last" time the unit was used, the indicated reading for a bullet at the very same velocity can be off by tens of feet per second. Introducing just 1/4" of of screen spacing error makes 2.08% difference in the indicated reading, meaning that at 3,000 fps the indicated velocity would be 62 fps different from the same reading "yesterday". As a Chrony gets some age and wear on it, the hinge naturally gets a bit looser, which can lead to the error I described. Or, setting it on a slightly uneven surface can also cause one or both screens to tilt, thus changing the relationship between them and yielding a skewed velocity reading.

I actually own and have used a Chrony, although I now have it on permanent loan to a friend since I have a more reliable unit available to me. I think I paid 50 bucks for it, and it was (and still is) well worth the price. Just don't operate under the illusion that it is infallible. If you're careful to get everything "just right", then it yields useful, if not absolutely precise, information.

Because of the short distance between the screens, the "Chrony" is very suseptable to even slight conditions changes. Not a fault of the machine, but when you only have a fleeting mili-second to detect the bullet, the rounding in the math can produce some differences and as mentioned temp, humidity, and distance from the end of the barrel all have an effect. I would not lose any sleep over the 100fps as long as your accuracy stays you are good. The rifle is telling you it likes 57grs.

Thank you for all the input!

The power of this forum is just awesome, there is so much knowledge here and you all take the time to share. Once again thank you all!

There is obviously more to chronys than I could imagine. Probably several things that might have caused the differences in my measured velocities. Temperature was about the same but day one was bright and sunny, day two was cloudy with a bit of rain in the air. I did not put up the chrony in the same spot and I am fairly sure it was not perfectly level and I was not shooting through the exact center of it. Most likely the hinge is also a part of the problem, as I said this is a fairly old Shooting Chrony and the hinge does not stop in an exact position when you unfold it.

I do like getting the information a chrony might provide. Probably will try to get my hands on a Ohler 35 to use for my more serious load development and use the Shooting Chrony when a quick but perhaps not 100% reliable reading will suffice.

I guess I am a practical man and I think that a load giving me sub 1/2" groups can not be way of in extreme spread or standard deviation. A velocity difference of 100 fps (if in fact true) is also probably not important in any practical way when hunting.

Conclusion; I am happy with my 57 gr load and I am very happy knowing a lot more about chronys. Thank you!

Sometimes muzzle blast will alter the reading.
For a quick check back off another 10 feet and
remeasure.

To compare yesterdays chronograph reading with todays you must take care that the chronograph is set up correctly (older Chrony's that fold open must be opened the same exact amount on each set up) and is the same distance from the muzzle. Recording atmospheric conditions is also good but only the ambiant temperature of the ammuntin and shooting condidtions is meaningful at the short range from muzzle to screens.

Also understand that two test strings of the same ammo most likely will give a different ES and AV. Not much different but different none the less. Then you use of 3 and 5 shot test strings leaves a lot to be desired from a sample size confidence issue. The use of 3 shots initially just gives you a bit of information. The 5 shot groups give you just an idea of what a load might do. Most ballisticians understand that to have a degree of confidence in a load a minimum of 7 shots in a test string is needed. The munititons industry and SAAMI use 10 shot test strings as standard. Yes I know the 5 shot test string is most commomly used in magazines, forums, etc. but that does not make it statistically valid or best to use. The statistical validing of 3, 5, 7 and 10 shots applies not only to the AV, the ES, the SD but also to group size on target. A 3 shot group really tells little, a 5 shot group is a little better but a 10 shot group will tell you the real accuracy potential for that load. Some use 3 five shot groups which is fine but do not make the mistake of using the average group size as the accuracy "potential of the load". The real accuracy "potential" is the ES of the group. Otherwise the rifle may do better than the average or it may do worse and you'll not know which. Knowing the ES of the group size (called the cone of fire) is best.

I suggest you shoot a slow fire 10 shot string with each of the 57 and 58 gr loads. Compare the AV, the ES and the group size. That will tell, with a high degree of confidence, which load is best.

Larry Gibson

quote:

Shot to shot velocity variations, how much is to much?

I have found that with some rifles 30 fps. is critical and with others 100 fps. is not when we are talking repeatability. It also matters if you are talking hunting vs. paper punching et all. roger

On your original question...I believe shot-to-shot variations mean nothing if you find an accurate load, as you have. If there is a 100fps variation and a 1/2" group, I do not care about the variation at all. Test the 1/2" load at various distances and see if the group size remains consistant.