Accuracy vs Repeatability

06 June 2008, 22:06

Claret_Dabbler

Not entirely sure what you mean by that?

I would have thought that accuracy - as measured by group size - is a function or a product of repeatability?

If you perform the same function, whether it is case prep, powder throwing, bullet seating, or actually shooting the rifle, in an identical, "repeatable", manner - every time you carry out the action - surely accuracy should follow?

Of course if you selectively pick your good groups as your measure of accuracy - and ignore the not so good ones, you are only fooling yourself.

06 June 2008, 22:07

Bobby Tomek

The key is repeatable accuracy. Just because somone fired a group of 0.28 on one day doesn't mean that it will happen again. I thoroughly test my loads, and if the accuracy can be repeated for a minimum of 5 groups, then I proceed to work further with that particular recipe. More groups are then fired -- perhaps even with minor changes to seating depth, etc. -- before a load ever goes onto my shelf.

On the other hand: I've seen shooters take stock rifles, cheap scopes, wobbly rings and factory ammo and get lucky enough to plant 2 through the same hole -- and then proceed to call the rifle a one-holer despite the fact that they never fired a group like that before and probably never will again.

06 June 2008, 22:44

mstarling

Lots of ways to look at this ... if we remove the shooter from the list of variables, then group size becomes the best measure of merit because 1) the group can be moved to the intended point of impact by moving the sights, and 2) once the center of the group coincides with the aiming point then the mean distance from the center of the intended point of impact is lowest if the group is small.

Introducing the shooter as a variable, if groups remain small then the combination of rifle and shooter are accurate and repeatable (consistent).

06 June 2008, 23:40

Gaillo

Hi!

I was taught this in the field of machining, but it applies just as well to shooting.

Accuracy = How closely an achieved result comes to a previously determined "standard" result. An example would be how closely a bullet or group comes to actually hitting the center of the bullseye.

Precision = How "repeatable" a result is - even if it's not an accurate result. An example would be a 0.1" 10-shot group, even if it is 6" away from the bullseye at 100 yds.

As shooters, we ultimately strive for BOTH - a tight group centered on the bullseye if possible.

As shooters, we often call a small grouping rifle an "accurate" rifle, when what we REALLY mean (given the above definitions) is that it is a "precise" rifle that holds a tight group. It is more proper to say that our rifle is a "precise" rifle when it groups small - unless we have the impacts dialed in to target center as well, in which case "precise and accurate" would be the proper description to use.

07 June 2008, 03:39

graybird

quote:
Originally posted by Gaillo:
Hi!

I was taught this in the field of machining, but it applies just as well to shooting.

Accuracy = How closely an achieved result comes to a previously determined "standard" result. An example would be how closely a bullet or group comes to actually hitting the center of the bullseye.

Precision = How "repeatable" a result is - even if it's not an accurate result. An example would be a 0.1" 10-shot group, even if it is 6" away from the bullseye at 100 yds.

As shooters, we ultimately strive for BOTH - a tight group centered on the bullseye if possible.

As shooters, we often call a small grouping rifle an "accurate" rifle, when what we REALLY mean (given the above definitions) is that it is a "precise" rifle that holds a tight group. It is more proper to say that our rifle is a "precise" rifle when it groups small - unless we have the impacts dialed in to target center as well, in which case "precise and accurate" would be the proper description to use.

Couldn't have said it any better. When in grad school working on remote sensing projects, professors always used the bulls-eye as an example of accurate vs precise/precision.

An accurate rifle could place 4 shots each 2 inches from the bullseye, with each of the bullets in a different quadrant of the target or in four different corners of the target, it would be accurate but not precise, because once the distance of each shot on the x and y axis of the target is calculated then the result would be the rifle is accurate because all the bullets cancelling each other resulting in the average being centered in the bullseye; thereby, the rifle is accurate but not precise.

A precise rifle would put 4 shots within 0.25 inches but all shots would be in a certain quadrant of the bullseye target.

Many people would rather have a precise rifle as opposed to an accurate rifle.

07 June 2008, 04:21

bartsche

Thought provoking responses. Very nice indeed.

roger

07 June 2008, 07:01

TX Nimrod

Judging a rifle's "accuracy" (which actually means precision among shooters) by the smallest groups the rifle makes is the goofiest form of self delusion, yet we read about it all the time. "My rifle shoots 1/2 moa groups" usually means it once shot a group that small. In benchrest competition - where precision is usually the chief goal - shooters are judged by their aggregates. Aggregates are the averages of consecutive groups. I judge my rifles' accuracy with a minimum of four consecutive 5-shot groups averaged together for a 20-shot aggregate. Not the best four groups, but 20 shots fired consecutively. I may have one 1/4" group and three 1" groups for an aggregate of 13/16". My rifle is thus a 13/16" group shooter.

.

07 June 2008, 08:20

mstarling

Fellas,

Please note that in the case of a rifle ... the standard definition of "accuracy" doesn't work well because (as I pointed out) the impact point can be moved by moving the sights [adjusting the scope].

If a rifle/sighting combination works at all well ... its impact point can be adjusted to that desired. Therefore, what is traditionally described as precision becomes the best measure of merit.

p.s. I've taught the same stuff too ... to scientists.

07 June 2008, 08:57

woods

quote:
Originally posted by bartsche:
Does anyone besides me have a problem with using group size to define accuracy rather than repeatability? roger

Hey bartsche

I think my reloading more closely follows the definition of insanity:

"Doing the same thing over and over again and expecting a different result"

____________________________________
There are those who would misteach us that to stick in a rut is consistency - and a virtue, and that to climb out of the rut is inconsistency - and a vice.
- Mark Twain |

Chinese Proverb: When someone shares something of value with you and you benefit from it, you have a moral obligation to share it with others.

___________________________________

07 June 2008, 10:18

Lamar

the way i check re-peatability is to sight in then shoot a group on a clean target.
i then take that target with me each time i shoot and put it behind a blank target.
after shooting in sun rain snow clouds hot and cold.
about 10, different 5 shot groups...
you will learn about re-peatability.
i once had a rifle that would shoot 1-1/2"
groups at 100 yds but it did it everytime i shot it, no matter the weather with no p.o.i.
change.
guess what rifle went with me no matter what.
the only reason i don't still have it is the one i have now is more accurate [smaller groups]
and is just as re-peatable.

07 June 2008, 10:24

303Guy

The secret to good accuracy with a rifle is to keep firing three shot groups untill an accidental sub-MOA group is obtained. For sigting in, accurately measure the distance from the target using a laser rangefinder then fire one shot. Adjust the scope and fire another. Repeat above untill one shot accidently hits the bulls eye. Then go hunting deer. I did seen it on TV!

PS - for repeately getting sub-minute groups, shoot one shot each!

07 June 2008, 10:31

303Guy

I use a system of 'accuracy' measure using a 75% probability and a 100% probability. I'm happy with a 75% one MOA probalility and a 100% one and a half MOA probablity. I hate a 75% one MOA and a 100% two and a half MOA. It means I will miss one out of four shots.

Bad odds and something wrong.

08 June 2008, 16:35

Red C.

For hunting purposes I want my first shot cold to be in the same spot as my 3 or 5 shot group that I was so proud of was the last time I was at the range. If my gun won't shoot within the 5 shot group, that bothers me. I want to know what my rifle is going to do that one shot on a hunting trip. To me, that's repeatability and accuracy as well.

Good, thought provoking thread.

08 June 2008, 17:21

vapodog

quote:
Originally posted by Gaillo:
Hi!

I was taught this in the field of machining, but it applies just as well to shooting.

Accuracy = How closely an achieved result comes to a previously determined "standard" result. An example would be how closely a bullet or group comes to actually hitting the center of the bullseye.

Precision = How "repeatable" a result is - even if it's not an accurate result. An example would be a 0.1" 10-shot group, even if it is 6" away from the bullseye at 100 yds.

As shooters, we ultimately strive for BOTH - a tight group centered on the bullseye if possible.

As shooters, we often call a small grouping rifle an "accurate" rifle, when what we REALLY mean (given the above definitions) is that it is a "precise" rifle that holds a tight group. It is more proper to say that our rifle is a "precise" rifle when it groups small - unless we have the impacts dialed in to target center as well, in which case "precise and accurate" would be the proper description to use.

Excellent post.....

As far as I'm concerned group size is the #1 measurement for a rifle.

09 June 2008, 12:45

303Guy

quote:
For hunting purposes I want my first shot cold to be in the same spot as my 3 or 5 shot group that I was so proud of was the last time I was at the range. If my gun won't shoot within the 5 shot group, that bothers me. I want to know what my rifle is going to do that one shot on a hunting trip. To me, that's repeatability and accuracy as well.

Well said - that about sums it up in my mind!

09 June 2008, 18:49

Hot Core

Hey Bartsche, I really prefer consistently repeatable precision accuracy for my Hunting Loads.

That is why I do the "Cumulative 1-Shot Groups" from an ambient temperature, pristine clean, lightly lubed bore to verify the above.
-----

quote:
Originally whammed in by Bartsche:
Does anyone besides me have a problem with using group size to define accuracy rather than repeatability?

How would seeing consistently non-repeatable accuracy be usable?

The only thing I can think of is it would probably tell you to get rid of the Mousers and M70s.

10 June 2008, 05:15

tom holland

Once I get a load I like to shoot it at 100,200 and 300yds some loads may shoot good at 100 then fall apart at longer yardage. I'll spread my groups out maybe over a month or two of shooting and my final shooting is one shot out of a cool barrel at 100yds then a follow up shots next day or so same thing at 200yd then later at 300yds. I figure maybe 150 rds to get a good deer/elk load.

Being an ex BR shooter group/agg you cann't have one without the other. To me the best shooter doesn't have to be a Br guy it's someone who may only be able to get a 2" groups out of his factory rifle but he get's that 2" group almost every time and he understands the capability of his rifle.

12 June 2008, 02:39

daveo

I shoot 10 round groups and just check to see if the first round fly's

12 June 2008, 14:14

303Guy

daveo, how often do you get a first shot 'flyer'? With a ten shot group, do you notice a pattern like 75% falling within a certain area and the remaining falling outside? I only used to shoot 10 shot groups, but found they consume quite a lot of ammo.

And with cooling the barrel between each shot, they take quite some time. (Plus, they open up the groups a bit).