Shooting Inch(s) Group

[jackadew]

My point is that you can't tell how accurate a given gun is (mechanically) with only a 3-5 shot group any more than you can accurately calculate the average height of the kids in a high school by pulling 3 of them at random and measuring.

The sample size is just too small.

If you want to demonstrate accuracy (in a statistically valid fashion) with only 3 shots, then they essentially have to all be single hole or darn close.

Follow me through here with an example (slightly different methodology, but you'll see it doesn't matter).


Let's say you shoot three shots and measure the distances from a center pinpoint which was your point of aim. Your results:
Shot 1: .5"
Shot 2: .25"
Shot 3: .25"

One would be tempted to say that you have an average error of .33", so not too bad. But these three shots are just a sample of thousands of possible locations and outcomes. What we're trying to discover is not how good a single group is, but what is the likely error of EVERY shot? What would a 3-shot group that seems pretty good tell us about what we could expect a 100-shot group to look like?


If you analyze this using statistics that account for the small sample, you find that the true average of all rounds (based on this sample) could be anywhere from 0 (an infinitely small error-- perfect accuracy) and 0.69". All values between 0 and 0.69" are equally likely (95%) to be the true average of all the rounds you will shoot. Think of this as the center of your group relative to the center of the target. Would you adjust your sights or not? How can you tell if you are dead center or off by over a half inch? You can't.

Then there's the spread within the group- or the average distance from the center of the group to each actual hole. This average error ranges from 0.075 (3/40ths of an inch) up to almost a full inch.

So here's what your three shot group tells you about what 100 rounds might look like:

-- There's a 95% chance that the 100 round group might be centered on the bullseye, or off .69"
-- There's a 95% chance that the 100 round group would have 68 rounds within 0.08" or within 0.9"
--There's a 95% chance that the 100 round group would have 95 rounds within 0.16" or within 1.8"
-- There's a 95% chance that the 100 round group would have 99 rounds within 0.24" or within 2.7"

So, the reality is that your 3-shot group indicated you MIGHT have a gun capable of 1/4" groups all day long, OR you might have a gun that shoots almost 3" groups all day at the same distance. They are equally likely.


Where I come from a 3" group and a 1/4" are different enough that I'd say that if my "testing" says they are equally likely, then I need a better test.

More info here: 68-95-99.7 rule - Wikipedia, the free encyclopedia

I get your need to use stats. But your missing the point behind why most guys measure groups. Why? Because if you shoot 100 shot groups with some larger calibers you could ruin the barrel from overheating for one reason.

When i shoot groups its usually to test ammo...no need to shoot more than 5 of each type ammo to tell me what i need to know.

I also shoot all my loads thru a chronograph and use a Kestrel weather meter. I record, group size, load data, velocity of each round, and all the weather details in a Shooting Log Book. So i can look back thru my Log Book and tell you how my guns performed with a given load and weather condictions...much more useful info for me.

 

[Hookeye]

100 shots as a number might mean something statistically, but it's more than just a numbers game.

Barrel fouling, heat effects etc.

I get repeatable small groups cleaning every so often and firing foulers and then going for group.
Usually weather influences my cleaning moreso than roundcount (as I hunt in all sorts of weather).

Worst case for round count came when shooting rifled choke tube shotgun with foster slugs, it leaded up badly,
A 20 or so shot group would be 7" or so at 75 yards. It started having fliers around shot number 15, after firing
2 foulers (inclusive in the 15).

With a shoot 2, then ten more, clean and shoot 2, ten more, we got it to shoot repeatbly at 3" at 75 yards.
The groups cut in half following that routine.

Even with my sabot slugs and rifled bores I shoot a couple of foulers and then clean at the 15 round mark.
Doing that got me under 2" at 100 yards, repeatably. No, I never shot 30 or so slugs without cleaning, just carried over the cleaning interval found on another rig. It worked great enough i saw no reason to test it to it's limits.

Varmint rifles get a couple of shots after a cleaning, then maybe a box or two before I get rained on and have to tear everything apart to oil/clean.
Action removal from stock and put back could cause way more noise than round count variance statistically.

There's a lot more to this game than just counting. You gotta apply it correctly to a system, just can't pick some arbitrary number. Yeah that will tell you something, what a gun shoots for 100 rounds. But you could screw it up huge if shooting dirty powders, fouling bullets.......or start with an oily bore vs dry, dirty vs a clean one.

Science and stats are fine but they need to be applied correctly.

Proper experimentation means very few variables and extremely detailed and repeatable processes............if one wants to claim it of some engineering merit (IMHO).

 

[Hohn]

You're right. A larger sample has its own challenges.

But there are ways around that. You can shoot one round per 30 seconds or so. You can clean every 50 rounds.

I'm not saying 100 rounds is sufficient to prove that a gun is accurate (nor that shooting 100 rounds straight proves anything, as it has its own variables).

What I *am* saying is that a 3-5 shot group proves next to nothing about the gun or ammo. Just the shooter, maybe.

 

[shibumiseeker]

You're right. A larger sample has its own challenges.

But there are ways around that. You can shoot one round per 30 seconds or so. You can clean every 50 rounds.

I'm not saying 100 rounds is sufficient to prove that a gun is accurate (nor that shooting 100 rounds straight proves anything, as it has its own variables).

What I *am* saying is that a 3-5 shot group proves next to nothing about the gun or ammo. Just the shooter, maybe.

The better analysis that I use is collecting the statistics of ALL of the groups I shoot with a particular gun. That gives me a pretty accurate view of what I and the gun/ammo combination is capable of. I can collect the min/max and all the analysis in between.

 

[Hohn]

That's a good approach, Shibum. That way you also average out the effects variation in a bunch of environmental variables: barrel temp, ammo lot, weather, etc etc.

 

[BE Mike]

You're right. A larger sample has its own challenges.

But there are ways around that. You can shoot one round per 30 seconds or so. You can clean every 50 rounds.

I'm not saying 100 rounds is sufficient to prove that a gun is accurate (nor that shooting 100 rounds straight proves anything, as it has its own variables).

What I *am* saying is that a 3-5 shot group proves next to nothing about the gun or ammo. Just the shooter, maybe.

I have a question. What gun did you last test for accuracy, how many shots did you fire and what was the group size?

 

[HighStrung]

Groups for me tend to be either 5 rounds or 10 after a couple fouling shots thru a clean barrel. Luckily, my 10-22bench gun has a trend, on a cold barrel the first shot is just almost always 1" low and 3/4" right with the ammo I've found it to like (other ammo's vary dramatically on a cold barrel though). I've gotten use to this over time and just know where to aim on my first shot. The following shots are very tight (dime size or less) grouped but impact at point of aim.

My question I guess, if I'm checking to see how good the grouping of my rifle is, do I need to aim at the exact same point for all shots, or am I okay with adjusting point of aim for what I know will be that flyer from the first shot on a cold barrel? By adjusting my point of aim for the first one I can put them all under a dime, I just didn't know if it was generally accepted to aim other than at the exact same spot for each shot.

 

[shibumiseeker]

My question I guess, if I'm checking to see how good the grouping of my rifle is, do I need to aim at the exact same point for all shots, or am I okay with adjusting point of aim for what I know will be that flyer from the first shot on a cold barrel? By adjusting my point of aim for the first one I can put them all under a dime, I just didn't know if it was generally accepted to aim other than at the exact same spot for each shot.

There really is no right or wrong there. This is a good example of knowing your gun and knowing your gun is a part of accuracy.

A poor gun would have that cold zero all over the place. Having a reliable cold zero is a very good quality.

 

[indyjoe]

I haven't taken my .357 Maximum hunting yet, because I haven't had the range time in cold temps to evaluate both cold bore accuracy and a load which groups good with that.

It takes a long time to get cold bore groups. I'm usually testing muzzle loader loads and shooting something else at the same time.