I'm trying to work up a few good loads for my LR rifles, and generally use 5 shot groups for accuracy checks. How many shots should I shoot minimum to check the ES of a load's velocity? I assume more than 5 shots. From what I've been reading, 20fps ES or under is what I'm trying for.... correct? Thanks!
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When checking ES....How many in string?
- Thread starter Fatelvis
- Start date
Ignore ES, it is not really a useful number. To get statistically meaningful info, you need 20 rounds, giving you a good average velocity and the standard deviation (SD), which is what you really want to know. An SD of < 15 is desirable, single digits is great, but given the precision of most chronographs, it's suspect anyway.
+1 on that. I've just got a MagnetoSpeed and that's an excellent bit of kit.
https://dl.dropboxusercontent.com/u/12296998/Magnetospeed/GAT Rifles 260 Rem + MagnetoSpeed.jpg
Regards JCS
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I shoot at least 10 but 20 would be better to get an overall picture of what's going on.
I use an Oehler now but with everything I have been hearing about the Magneto I have picked one of them up to try. Will be much easier than setting up sky screens.
I use an Oehler now but with everything I have been hearing about the Magneto I have picked one of them up to try. Will be much easier than setting up sky screens.
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Small strings are OK for looking at speed progression during a load workup but it takes a lot more to obtain meaningful calcs.
When I'm satisfied with a particular load i then shoot a minimum of 25 rounds over the chronograph. Sometimes I'll shoot an entire day's practice session over it. When you have 25-50 shots worth of data you'll see how meaningless the ES number is. With that amount of data the Mean Absolute Deviation. This is the average deviation of all shots above or below the average velocity of all shots. If your chronograph doesn't calculate this then hit the "net". There are all kinds of free "calculators" for MAD. Just enter all your speeds and let the bit's n bytes do the work.
If you happen to have ONE round that is 100 fps slow, for whatever the reason, the ES will be 100. If the other 20+ rounds are right close to each other, just what did that high ES mean? A lot less when you shoot a higher number of rounds for speed than if you only rely on 5 or so.
When I'm satisfied with a particular load i then shoot a minimum of 25 rounds over the chronograph. Sometimes I'll shoot an entire day's practice session over it. When you have 25-50 shots worth of data you'll see how meaningless the ES number is. With that amount of data the Mean Absolute Deviation. This is the average deviation of all shots above or below the average velocity of all shots. If your chronograph doesn't calculate this then hit the "net". There are all kinds of free "calculators" for MAD. Just enter all your speeds and let the bit's n bytes do the work.
If you happen to have ONE round that is 100 fps slow, for whatever the reason, the ES will be 100. If the other 20+ rounds are right close to each other, just what did that high ES mean? A lot less when you shoot a higher number of rounds for speed than if you only rely on 5 or so.
If you were paying attention to your shots then you can take the anomaly of the 100fps difference load out and your ES will be more where it should be.
It's good to look at both ES and SD when working up a load to get a full picture of what you are getting as well as getting your average velocity to punch into your ballistic program.
It's good to look at both ES and SD when working up a load to get a full picture of what you are getting as well as getting your average velocity to punch into your ballistic program.
I just got done using my Magneto today for the first time. It worked flawlessly. My loads that have the bullets jammed definately had lower SDs.
When you're testing a string and the SD is low (<15fps) and you get one of those really strange readings that is >100 fps off from the normal range then you need to consider whether that abnormal reading should be discarded from the sample or left in when you run your stats. The whole purpose of the speed measurement is to have an accurate estimate of what a cartridge will likely do next time you shoot, so you can accurately calculate the ballistic performance based on the expected speed. Don't throw out a measurement from your sample because you don't like it, or it isn't as 'good' as you'd like; but throw it out if it probable that it was inaccurately measured or it is unlikely to occur again.
With some of the optical chronographs, you can get really unusual variances due to changes in ambient light (ever have 1 shot in a string not register at all?). You also want to consider if there was some anomaly in the loaded cartridge itself that would make it unrepresentative of your likely future cartridge (e.g. was it the first case loaded from a chargemaster after changing powders?, had it been seated in a hot chamber longer than your other rounds?, etc).
With some of the optical chronographs, you can get really unusual variances due to changes in ambient light (ever have 1 shot in a string not register at all?). You also want to consider if there was some anomaly in the loaded cartridge itself that would make it unrepresentative of your likely future cartridge (e.g. was it the first case loaded from a chargemaster after changing powders?, had it been seated in a hot chamber longer than your other rounds?, etc).
+1. I did a dry run in the dining room getting it all set up. First outing in the field on a grey damp windy morning was a revelation.
Regards
JCS
