Number of shots/groups for feeling certain of your rifle's accuracy?

[horseman308]

So I read a comment about number of shots per group to gauge accuracy.

. . . your 3-shot group's got no more than 20% confidence of being what all shots fired will do. But it's very good for a factory rifle. Shoot another one with 10 shots and that'll be about 60% on the confidence scale. - Bart B.

From that I ask myself (and y'all) where we got the idea that a given 10-shot, or 5-shot, or even 3-shot group would give a good idea of a rifle's accuracy. What I mean is, why would we start to believe relatively few rounds would be a valid indicator?

In grad school, when I had to learn statistics and how to conduct research, one of the basic things was that you need plenty of observations of a thing before you can say that, on average, your conclusions are valid. Statistically, an average based on a minimum of 30 observations gives tolerable validity. So based on that notion, backed up by Bart's comment, I'd want at least three 10-shot groups, or six 5-shot groups, etc. (or better yet, 30 groups of 10 shots), taking the average group size, to say with real confidence that my rifle has a certain amount of accuracy.

I figure most would agree that more shots is better than fewer when testing accuracy. Obviously, that many rounds gets expensive and time consuming, especially if we're talking about match-grade ammo. Still, is there any other reason that so many people rely on a relatively small number of rounds? Just curious.

 

[Brian Pfleuger]

It depends on what you want to know.... 3 shots tells me what I want to know, the vast majority of the time.

For instance, I don't really want to know MOA, the majority of the time, I want to know how far from the POA I'm likely to hit and/or I want to sight in my gun.

"Statistically valid" is different than "It tells me what I want to know."

I sight my gun in with 3 shot groups. I've never known to miss because of it, targets as small as a woodchuck at near on 400 yards...

Besides which, "statistically valid" is a waste of time and ammo, for me, and doesn't tell me anything that helps me shoot better.

 

[sigcurious]

I would also suspect that collectively, enough x-shot groups have been shot enough times for the consensus to be that if the shooter and other variables are out of the equation an x-shot group is a good indicator of a rifles accuracy and precision. Sure there may be those times where someone has an inherently inaccurate and imprecise rifle, that lucks out into a good group, but they probably figure that out with subsequent shots and or groups.

 

[NWPilgrim]

For me there are two different situations. One is doing comparative accuracy when working up test loads of a powder and bullet combination for a rifle. I typically load 5 different powder charges to try to find the one that will be the most accurate. For testing handload work ups I shoot five shot groups for each powder load. That is 25 rounds just for one powder/bullet combination. Figure I may test several powders and bullets for one caliber and it starts to add up quite a bit. For me a five shot group gives reasonable RELATIVE accuracy between the loadings. I can easily see the groups tighten up and then open up progressively with powder increases.

The second situation is to determine the full accuracy of myself and a rifle/bullet/powder combination. In this case I agree that it takes a larger sampling to give confident results, but I am not going to sit down and shoot 30 groups to find out. Rather, I will see how a chosen load performs over time. For one thing, I am rarely going to fire groups of more than 8 or ten rounds at a time. Typically these will be a bit larger than the five shot group but I have never seen them open up dramatically compared to the 5 shot groups. It seems if a load is 3/4" group with five shots it may increase to 1 1/4" with ten shots, but it is still the best group of the different loads.

And then there is the human factor. My shooting starts to degrade after a while. If I shoot too many rounds for a test then the latter groups will open up just from fatigue. So I try for a balance of what is practical given my limits of budget and ability and the largest sample size. It may not be 100% but it works for my purposes. At some point of accuracy you get to where the shooter is more of a limitation than the equipment.

Just as a comparison, my dad was an old Pacific theater Marine. He easily qualified expert in training and was a natural rifle shooter. He would sight in each hunting season with 2-3 rounds to confirm zero. He used basic factory SP ammo. And I never knew him to miss or lose a deer or elk. That method would not work for a benchrest shooter, but for him and his hunting it worked well.

 

[JD0x0]

like every scientific experiment, the more data you have, the better your answer will be.

a 3-5 shot group would be more than adequate, to understand the rifle's basic accuracy, but 20 shots could tell you more about the rifle's behavior. Like overall shot consistency, or how fouling and/or heat affects your groups.

 

[5R milspec]

I have come to like a 3 shot group for testing.I have been for sometime now testing with 3 shot groups while working up in my powder charge.When I find the charge I want I then see what 10 will do 2/3 times with different lengths of the loaded round,and what ever load shoots the best.I then try it again to make sure the load,rifle and me are all working on the same page.So far this way of doing things has made me some pretty good ammo.

 

[Sweet Shooter]

That comment was made about a group I had shot with my bone-stock, off the shelf S2. If you look at the photo you will see that the group is clover-leafed and slightly high but near perfect L|R. I don't need to shoot 10 shot groups with it, as Bart suggests, because it does this every time I shoot it. I have about 200 rounds through it and the have owned it from new. IIRC it's shot a couple of groups that were just over an inch in the first 10 to 15 groups so was not really broken in.

The way I judge it is not only the group size but the consistency in where it actually prints that group. If it prints two .5MOA groups but one is off to the left and one is a bit high then, I'm not happy. If I can move that cloverleaf within the black by adjusting the scope and then it returns when I revers the process then I'm good with it (It's a VX3). To me the scope is more important than the rifle to shoot this kind of accuracy.

I can make my rifle shoot bigger groups by shooting 10 shot groups and getting it hot. That does not make it any less of an accurate gun.

http://thefiringline.com/forums/attachment.php?attachmentid=86842&d=1360282283

-SS-

 

[jmr40]

A single group, whether 3 shots, 5 shots, or even 10 shots does not tell me much. I'm certain of my guns accuacy after about a year of ownership. I rarely shoot more than 3 shots, but multiple 3 shot groups fired over several months and multiple range trips are what I'm concerned with. I'll never fire more than 2-3 shots at a game animal anyway so I'm not interested in 5 shot groups. I am interested in how well it fires the 1st 3 shots from a cold barrel after setting in the safe for weeks, and doing it time after time.

 

[PawPaw]

I agree, the larger the sample the better my confidence in the prediction might be. However, there's no sense wasting time or effort on something that doesn't work. If I start with load development and have a ladder set up, it's easy to tell with 3-shot groups what might work and what might not. If it doesn't work with a 3-shot group, it isn't going to work with a 10-shot group.

After that initial test, I start using larger group sizes, until August. Then, I start firing one-shot groups from hunting positions. Often, I'll take my rifle to the range and fire one shot from the cold bore. Then I put the rifle away and work with pistols or another rifle. I want to know with certainty where that one cold-bore shot is going to hit.

 

[sunaj]

In most guns your point of impact starts to change due to the heat distorting the barrel,
so you take note of your first groups and learn where your point of impact changes after firing several rounds (which is why some keep changing their scope settings at the range and are scratching their heads-"whats wrong with my scope???")
sunaj

 

[hooligan1]

I'll weigh in here, I don't usually build over three different powder charge weights when testing, but I will load the next three if my testing is inconclusive. When, and only when, I find the one powder charge that is consistant and repeatable, will I load a quanity of over five cartridges, and I ususally load fifty, then I practice, sight in, shoot off my tri-pod trigger sticks, and such.
I could go to the range and spend fifty rounds for a said caliber, but I don't shoot up ammo aimlessly, I can't afford it. (and I like to stockpile).
But when the load Is named "The One" then believe you me I can make nice groups on targets, no problem.

 

[Striker1]

Do you guys shoot differing round count strings for different type rifles? Say a heavier barrel VS a sporter profile.

 

[tahoe2]

a full magazine, whatever that is 3,4,5, or 6. different guns have diff capacity, after one magazine full the barrel gets too hot and shots begin to wander.

 

[603Country]

My approach is the very same that PawPaw uses, though for some recent shooting I used 15 shot groups.

On my usual approach of 3 shot groups, when I find a group that looks promising I'll go load up a few more and make it a 5 shot (or more) group. Lucky for me, my shooting bench and reloading bench are in the same barn workshop. This time of the year, while I'm making more ammo, the barrel is cooling.

And once I find the load I like, whenever the mood strikes me, I'll go shoot one cold bore round to see where that hits, because that's how it'll work when I'm hunting.

 

[reynolds357]

The cold bore round is the only one I care about in a big game hunting rifle. The yote rifle needs to shoot good warm, but very rarely do I shoot more than one deer a day.

 

[PawPaw]

On the other hand, over the course of a summer, I built a composite target one one rifle. When I'd get home after a range trip, I'd drag out my targets and overlay them on to a single clean target. That composite target had nine different handloads on it, with five different bullets and over 200 shots. Every shot, every flier, every bullet weight fell into a 3" group.

 

[PatientWolf]

I would argue that to sit down and fire 30 or 50 shots in a row in many cases would not be statistically valid since that may not be how the rifle would be used. The heat build-up would that would not exist if you fire 1-3 shots would be an additional variable. If you are not satisfied with a 3 shot group, but that is what you would likely shoot, you would probably need to measure and analyze the first 3-shot groups from multiple days, in which scenario, you now need to add weather as a variable in most cases.

Me? I would be happy with a few 3-shot groups under a MOA, and more than happy the closer I get to 1/4 MOA @ 100 or 200 yds. And not try to overanalyze it.

 

[Bart B.]

Rifles that move point of impact relative to the aiming point as the barrels heat up have at lease one of three problems.

* The barrel's not fit correctly to the receiver; it bears hard at one point around its tenon shoulder because the receiver face ain't square with the tenon threads.

* The barrel's not made of good steel.

* The barrel's not stress relieved properly.

Top quality match barrels fit to squared up receivers will shoot dozens of shots 20 seconds apart into sub 1/2 MOA groups at long range; sub 1/4 inch groups at 100 yards.

Most factory rifles could be cured if their receiver face was squared up and a metal shim put between the barrel tenon shoulder and receiver so it would clock in correctly.

 

[reynolds357]

Bart, most hunting rifles do slightly move POI from cold barrel to warm barrel. I am sure you are correct in why they do so. It does not bother me because most hunting rifles are not built to match specs. I also dont have any hunting rifles that cost what my action on my best shooting bench rest rifle cost.

 

[Guv]

Agree with you 357 bout the cold barrel. I can add for myself that also a barrel with less than 20 rounds through it since a good cleaning, while cold would be my first choice. 4, 3 shot groups should be plenty. 5 shot groups are for a different purpose than just sighting in a hunting rifle, can work out well for braggin though!