Std Deviation - what is a good value

[brianmahler10]

What std deviations are y'all typically seeing on your reloads?
What should I expect? What is considered good?

I ask cause I'm seeing 50fps and sometimes 100+fps.

 

[bultinkle]

How many rounds?

 

[Varmint Hunter]

IMO - good SDs are in the single digits (<10fps), although they aren't necessary for good groups at typical hunting distances of less than 400yds.

My bench guns will often land in the very low numbers (1-2-3 fps SD)

 

[bultinkle]

IMO - good SDs are in the single digits (<10fps), although they aren't necessary for good groups at typical hunting distances of less than 400yds.

My bench guns will often land in the very low numbers (1-2-3 fps SD)

How many shots are you using when you get 2 fps SD

 

[Phil Rizzo]

AND….be aware that sometime your load with the tightest ES won't necessarily be the most accurate on paper.

i don't know your cartridge, but high load density generally promotes lower ES-SD.

 

[Tiny Tim]

What std deviations are y'all typically seeing on your reloads?
What should I expect? What is considered good?

I ask cause I'm seeing 50fps and sometimes 100+fps.

Are these values ES or SD? Single digits to mid teens are typically achievable once a barrel is broken in. The more overbore a cartridge is, it seems more difficult to achieve.

Minimum sample size of 10 rounds and 50 is far better. I usually have my chronograph with me whenever I'm at the range, so a larger sample size in differing conditions is very valuable for hunting purposes.

Agree with Varmint Hunter, not necessary for 400ish yards.

 

[The Oregonian]

Basically what Std dev mean is that ~66% chance the velocity will fall within one std dev from the mean, ~95% that it falls within 2, and ~99% chance it falls within 3 std devs of the mean.

That may help people put into context what it means.

 

[Varmint Hunter]

How many shots are you using when you get 2 fps SD

Typically 3 or 5 shot groups. The new Garmin produced a SD of .1 the other day when I was shooting a 6mmBR. On a few extremely rare occasions, my LabRadar would produce a SD of ZERO.

 

[Varmint Hunter]

AND….be aware that sometime your load with the tightest ES won't necessarily be the most accurate on paper.

i don't know your cartridge, but high load density generally promotes lower ES-SD.

Phil is on the money with this one. I have noticed that groups with the smallest SDs rarely produced the smallest groups. It's probably just a coincidence but I have seen it happen on numerous occasions. Low SDs are desirable but they don't necessarily equate to the best tune for the rifle.

 

[Idaho Lefty]

Plus or, Minus 7, gets my Tikka T-3, 24", .270 WSM with, 140 Berger Class Hunt's @ 3,173 FPS ave Vel., Sub, 5/8ths, MOA, when I do my "Part".
Like to see, even Less ES, from my 6 XC with, 107 SMK's that are lightly jammed = BugHoles in, the 2's and 3's.
Matching Bullet to, the Twist and Velocity ( correct, Powder Pick ) for accuracy, is my main goal.

 

[memtb]

What std deviations are y'all typically seeing on your reloads?
What should I expect? What is considered good?

I ask cause I'm seeing 50fps and sometimes 100+fps.

Those #'s seem a bit high to me…..I wouldn't be happy with that!

However, as others have indicate, at closer ranges, say 400 and under, it's likely "not" a game changer"! At longer ranges, a deviation of that magnitude with make your groups exponentially larger as ranges increase!

"On the bright side", with those large deviations, what would have been a big miss at long range due to "operator error"……may result in a hit! Yes…."my glass is running over"! memtb

 

[Tiny Tim]

Phil is on the money with this one. I have noticed that groups with the smallest SDs rarely produced the smallest groups. It's probably just a coincidence but I have seen it happen on numerous occasions. Low SDs are desirable but they don't necessarily equate to the best tune for the rifle.

Great point! At some point it may be a balancing act where at distance a high ES will produce a more detrimental effect on group size than a less precise load with a lower ES/SD.

 

[Tiny Tim]

Those #'s seem a bit high to me…..I wouldn't be happy with that!

However, as others have indicate, at closer ranges, say 400 and under, it's likely "not" a game changer"! At longer ranges, a deviation of that magnitude with make your groups exponentially larger as ranges increase! memtb

Agreed, that's why I asked if he meant es. What kind of ES does it take to produce a SD of 50-100+ fps?

 

[bultinkle]

Looking at my garmin app On 15-20 shot groups I'm looking at SD around 8-10 for my rifles and high teens low 20s on my dads semi auto that's not shooting the powder I would chose for a bolt gun.

The one 3 shot group I have recorded is 3.6fps SD from a tikka barrel.

I personally put very little into it, but 50-100 seems like a lot to me and is more like an ES

100fps SD is pretty wild velocity swings.