What is a good standard deviation for my reloads?

Theoretically a very small SD is what we want.....but in fact it's not truly closely related to accuracy.

Un;ess you're a very long range bench shooter, I'd focus on the groups and forget the standard deviation.

IMO SD under 15 is fairly good.....but as I said....not terribly valuable.

d,

You have asked a question with a complicated answer. Just so we are on the same page, I'll define standard deviation (SD) as velocity uniformity and that we are talking rifles not handguns. I pay close attention to SD and I shoot twenty-five rounds to determine SD for a load. It is possible (in my experience rare) to shot a large group with a single digit SD. If so, this is telling you powder burning is uniform but the charge weight is not the one that works with your barrel harmonics. SD can provide insight into why you get good 100 yard groups but group opens up at longer range. With a small SD you can correct sight-in hunting around the globe with confidence.

To not-answer your question, SD provides insight to why a bullet behaves and should be used as a comparison value to other load development for your rifle. In carefully prepared loads for my hunting rifles I have single-digit SD.

vapodog has it right. Look at the target and not at the chrono. While, in theory, 5 rounds showing low SD should group better than 5 rounds showing worse SD, in practice this is often not the case. Too many other variables and not 100% efficiency.
Peter.

I also found that SD is not worth worrying about.When shooting High Power Service Rifle one of my better grouping loads for accuracy out to 600 yds.,had the largest SD. Almost 65fps .

A standard deviation of zero is better than a larger number -- all other things being equal.

quote:

Originally posted by Hammer:
A standard deviation of zero is better than a larger number -- all other things being equal.

Yea, but what is the SD for your loads.

I shot 3 different loads for three different bullets in my 300-221 AR-15.

The 125gr load has a SD of 14.2
The 165gr load has a SD of 36.7
The 240gr (subsonic) load has an SD of 32

I consider 30 good. I don't like and I think that 32 at subsonic vel. may cause me issues at longer ranges.

Edited because I am a math idiot.

quote:

Originally posted by ddunn:
What is a good standard deviation for my reloads?

Lower is better but doesn't necessarily mean the load will be accurate. It does make me think that the cartridge is happy with the load.

My 9.3x62 has shown some incredible SDs (Low single digits) with a few loads. Not all of them were extremely accurate, but it makes me think there is some great potential for accuracy, or the cartridge is very well designed, or the ballistic gods are happy, or SOMETHING good is going on.

Good shooting,
desmobob

quote:

Originally posted by ddunn:
I shot 3 different loads for three different bullets in my 300-221 AR-15.

The 125gr load has a SD of 30
The 165gr load has a SD of 103
The 240gr (subsonic) load has an SD of 61

I consider 30 good. I don't like 103 and I think that 61 at subsonic vel. may cause me issues at longer ranges.

At the risk of opening a can of worms here, I would say sometimes it depends on the actual velocity values of each shot used to calculate the SD.

I had a friend in Edmonton who got really lousy groups from his .340 Weatherby, but the real problem wasn't revealed by the SD alone, but by the fact that at least one round in every five was about 225 fps slower than the rest. That definitely showed up on the target at 200 yards.

The cure for his problem was changing to another primer so the powder charge lit more consistently and burned more thoroughly.

So, if you get higher SDs than you like, I suggest the next step may be to try to evaluate what is causing those high SDs.

Some of my most accurate (on the target) loads have SDs over 40, but no really wild extreme variations.

Of course, for black powder rifle cartridges, an SD over 10 may mean there is something wrong with the load....

I think you guys have answered the question, I should be able to do better. I plan on/need to converting over to a different primer anyway. I also want to push the 165gr and 240gr a little faster (i.e. different powder for 165gr and more powder for 240gr)

Here are the details.
Remington 125gr SP
2128
2125
2155
2141
2155

165gr SP
1503
1606
1542
1552
1544

240gr SMK
872
904
894
972
910
933
881
893
868
875

d,

Here is Murphy's Law on standard deviation:

Large groups usally repeat;
Large groups with large standard deviations always repeat;
Small groups caused by luck never repeat.