Chronograph SD questions

[Gbar222]

I am still somewhat new to long range but expanding my knowledge and experience as much as I can. I just picked up a Garmin Xero chronograph with the intention of getting into reloading relatively soon. It’s an amazing unit and super easy to use, but I have a couple questions. What is the appropriate number of shots to measure for an average? What are considered good or bad SD and spread numbers?

I have measured a lot of groups across multiple rifles. Most of the Hornady and Norma factory ammo has come in with SD’s from 13-20. I recently measured 12 shots of 7PRC ammo from Choice Ammunition(Berger 168 VLDH). I was surprised to see SD at 45 with spread of 81. I then measured Hornady 175 ELDX Precision Hunter for 7 shots and got a 13.4 SD with 45 spread. The Choice groups well at 100 yards but I haven’t taken it past 400 yards yet.

What should I typically look for and consider good for factory ammo?

What is the right number of shots to base my averages off?

I appreciate any wisdom, best practices or advice that you can share. Thank you.

 

[Nik H]

The XERO is amazing because it makes it easy to collect accurate data.

I have a spreadsheet where I keep track of all the cartridges I shoot. I differentiate by Lot # and rifle. I record the environmental data at every session. I shoot a lot of FGMM 308 168 SMK and Hornady 6.5 CM 140 ELD. Since I buy a steady amount of ammo, I have lots of samples from the same lot for my most often-used ammunition. Therefore, I calculate SDs based on hundreds of rounds from the same lot.

I believe the SDs are meaningful even though it isn't shot at one time.

 

[Gbar222]

Thanks for the response. I’m keeping meticulous notes also. I’ve already seen some velocity changes with ambient temperature, nothing crazy but it’s definitely there. Should I bother using the 45 SD at distance or start looking for another load?

 

[Nik H]

Thanks for the response. I’m keeping meticulous notes also. I’ve already seen some velocity changes with ambient temperature, nothing crazy but it’s definitely there. Should I bother using the 45 SD at distance or start looking for another load?

I just got the XERO. I am a bit ADD, so the SDs are interesting.

The proof is in the pudding, though. I use the long-term average as a start. However, truing based on live data sometimes is necessary.

I find that for Federal 308 (multiple versions) and Lapua 308. Berger, Federal w/Berger, and Hornady ELD-M for 6.5 CM guarantee a positive outcome regardless of SDs.

 

[chevy_man]

How far are you shooting?

Ideally you want to be as tight as possible. But, it doesn't really matter if you're not shooting past 200 yards.


I only ever use my own hand loads. If the SD isn't under 10 and the ES isn't under 30 I tweak the load.

80fps is easily seen at 1000. It's not noticeable at 200.

 

[Matches Malone]

This ain’t garmin specific but to get a good sd you really want to shoot as many rounds as you can afford at it.

I would say 5 rounds would arguably be lowest count for usable information. 10 or more being ideal.

In regards to shooting as many rounds as you can afford. ES would be another one. The thing about ES is it will certainly never get better the more rounds you put in a string, so the more round a batch can withstand is a true test.

So for practical data, imo, SD can effect hit probability across the board, but I think ES will be more so a concern at distance beyond 600.

For more, read through this article. It will save time as opposed to typing something similar.

How Much Does SD Matter?

As long-range shooters, we tend to obsess over every little detail. After all, we’re trying to hit r...

precisionrifleblog.com

 

[Doom]

There is another series on PRB that talks about statistics in general. It may be of interest.

https://precisionrifleblog.com/2020/11/29/statistics-for-shooters/

 

[Rio Precision Gunworks]

So for practical data, imo, SD can effect hit probability across the board, but I think ES will be more so a concern at distance beyond 600.

Without typing out a short novel, this isn't how SD and ES work. Your ES is going to be 2x your SD for 68% of shots. 4x your SD for 27% (95% total) and 6x your SD for 4.7% (99.7% total).

Meaning that it's impossible for ES to be *more* a concern at a certain distance. Since your SD is going to be a much better indicator of what your ammo is capable of.

 

[Old Man with Gun]

Variance in velocity causes groups to string vertically at longer distances. So, understanding how it varies and trying to reduce it for your load is of value.

Let's look at two different ten shot string with same spread, both shoot between 2750 and 2800 so ES is 50.
The first string has 5 shots at 2750 and 5 shots at 2800. The group would be five rounds tightly clustered with another five rounds tightly clustered above it. The average velocity would be 2775 and the SD would be 26.35.

The second string has 9 shots at 2750 and 1 shots at 2800. The group would be nine rounds tightly clustered with a single round above it. The average velocity would be 2755 and the SD would be 15.81.

A lower SD represents a load where most of the velocities cluster very close to each other. A low ES represents how far off your flyers will be when you have them. You can't have a low SD without also having a low ES, but even when the ES of two loads are the same (as above), one load may cluster more shots together.

Because you are probably doing your load development at a much shorter range than where vertical stringing becomes a big issue, you must rely upon chrono data and not groups to understand if the load will work at distance. The distance between 2750 and 2800 on paper at 100yds may be undetectable, whereas at 1000yds it is a big issue.

 

[mark5pt56]

One thing to look at is the first shot and any subsequent ones that may be off due to bore condition. Not to side track but that could be from cleaning methods, you can elect to exclude those from the numbers via the app or all together on the device, I don't so I still have the data.

 

[Gbar222]

How far are you shooting?

Ideally you want to be as tight as possible. But, it doesn't really matter if you're not shooting past 200 yards.


I only ever use my own hand loads. If the SD isn't under 10 and the ES isn't under 30 I tweak the load.

80fps is easily seen at 1000. It's not noticeable at 200.

I regularly shoot to 1000 with other calibers, but I haven’t taken the 7PRC that far with the 45 SD 81 ES load because I thought it could be an issue. I wasn’t sure that I could trust the DOPE with inconsistent ammo. For 1000 yards, what is considered respectable #’s for factory ammo and which brands typically deliver more consistent performance? Thank you for all of the responses here. I’m learning a lot.