Measuring Group Size

Des Moines, Iowa – (Ammoland.com)- Now that spring is upon us – really, it is! – shooters in the less temperate regions of the U.S. are dusting off their range bags, digging out the shootin’ irons, and heading out to the nearest range to brush up their skills. If you’re looking to improve your shooting, or just keep an accurate record of your and your guns’ performance, you need to know how to measure shot groups. The easiest and most commonly used method of measuring shot groups is the “center-to-center” method . . .

This means simply finding the distance between the centers of the bullet holes farthest apart on the target.

Finding the center-to-center measurement is done best with a caliper. Since we don’t usually require an ultra-precise, ten-thousandths-of-an-inch measurement for group size, an inexpensive caliper like the Lyman dial caliper (Brownells #539-832-212) is more than adequate for the job. If you don’t have a caliper though, a ruler that measures down to at least 1/16th of an inch will work just fine.

To begin finding group sizes, you must first know the diameter of the bullet you’re shooting. If you’re shooting a bullet designated in inches, like the .308 Winchester or .45 ACP, you’re already in business since the caliber is the diameter. If you’re shooting a metric bullet, like a 6mm or 9mm, you’ll need to do a little converting: 1 mm equals approximately 0.039″. So, for example, a 9mm bullet is about 0.35″ in diameter.

Once you know that, measure your group from the edges of the bullet holes farthest apart on your target. In the target at the top of this post, the calipers read 1.818″. This four shot group was made by a .308, so the next step is to subtract the bullet diameter from the group diameter. In this case we have 1.818 minus 0.308, which equals a group size of 1.51″, from the center of each of the farthest holes.

Here’s another example:

Measuring Group Size

In this instance, the calipers measure 0.834″, and the bullet was a .308 again. So we have 0.834 minus 0.308, for a group size of 0.53 inches, center-to-center.

It’s as simple as that! Measuring your groups will help you evaluate your own shooting for the day, but to take the most advantage of group sizes, you should keep track of them over time to see if you’re getting better or if you need to change something – or to track a gun’s accuracy. To do that, you can use a log book, like one of the Brownells Modular Data Books. During each range session, simply write down the group sizes you shoot, along with the firearm and cartridge information, and your distance from the target.

The Brownells data book even has a place to draw a sketch of the group as it appears on your target so you can track points of impact versus points of aim in various conditions. Collected over time, this data can help you measure your performance, the accuracy of your firearms and cartridges, the life of your barrels, and it can even help you diagnose problems – a topic for a future article.

As the saying goes, knowledge is power. When you start measuring your groups and keeping track of them, you’re collecting the data you need to start squeezing every ounce of performance from yourself and your firearms.

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46 Responses to Pro Tip: Measuring Group Size

  1. If my “group” at 15 yds with a pocket pistol is smaller than my hand, I call it “good.” If I can do a mag dump shooting as fast as I can with the same gun and they are all within the outline of the target figure, I call it “f*ckin’ amazing.” That’s my “measuring” lesson for today.

    • This is often called “combat accuracy”. It is a compromise between speed and group size. If your group size is too small (less than 4 inches) then you should speed up your shooting. If it is bigger than 6 to 8 inches, then you should slow down. All of this assumes your group is centered on the center of the target. If not, then measure from the center of the target to the farthest bullet hole.

      In a DGU situation, you need to get shots within center mass as fast as you can. Combat accuracy is good enough. In fact, it should be the goal of every gun owner who is training for self defense.

  2. Please stop recommending dial calipers. It is a disservice to folks who have no need to learn the obsolescent skill of reading a dial caliper. A Harbor Freight digital caliper is a superior choice for anyone who doesn’t already own precision calipers.

    • And you can automatically do center-to-center measurements with a digital caliper:

      1) Close the caliper completely

      2) Zero the caliper (to make sure you are starting at zero when the caliper is closed; you should basically develop a habit of re-zeroing a digital caliper often to make sure that your measurements are accurate and not impacted by an accidental re-zeroing)

      3) Open it to your bullet size (reading it directly in inches or millimeters; most digital calipers will do both)

      4) Zero the caliper again without moving it from your bullet diameter setting (thereby creating a negative offset the diameter of your bullet)

      5) Measure directly outside to outside across your group getting an accurate center-to-center distance

      One of the many digital caliper tricks…

      • I think it is silly to not learn how to use a non-digital caliper. Kinda like learning to drive an automatic car.

        So I would say it is far from obsolete, is it obsolete to learn how to use a bolt action even though we have automatics?

    • 10-4 on that! I have 4 pair of them.All Harbour freight. 4″ and severl 6″. The 4″ is a waste of money, too hard to read, and not any cheaper than the 6″. The last time I looked, they were only a sawbuck.

    • Not everything needs to be digital and require a battery. Simple math and a good analog caliper still gets the job done.

    • Nonsense.

      BTW he left out where you subtract the diameter of the bullet from the amount measured with the caliper via slide rule…

  3. Or measure from the right side of one bullet to the right side of the other. That way you don’t have to worry about converting metric to English. It’s like measuring from center to center, but you don’t have to find the center.

    • Yes a nice quick easy method with no math.

      Or hell, unless you’re Mr. Precision, just eyeball center to center, I doubt the most you’d be off is 0.1″

    • I was getting ready to say that, but, when you have a couple of holes overlapping, you must measure from the outside. When they don’t overlap though, that is a simpler way.

    • I fired 16 groups this morning at our small local range. First string I have fired out of the new Savage model 10 Precision carbine, with 20″ barrel, since I fire formed 100 new Lapua 223 cases last week.

      I only fired 3 shot groups, since I had a lot of groups to shoot, all with a different combination of loadings, some with different bullet placement in the case, and different powder weight, all Hodgon 322, and Berger 52 grain flat base bullets.

      I just use a scale with 10ths., and 100ths. The kind you would find in most machine shops, since I was not shooting in a match, and .005″ didn’t make any difference in this case.

      All my groups were under 1.000″. Six groups were under .500, and 4 were under .375 The total average for 16 groups was .458″

      So now I will load 5 shot groups of the best 3 shot groups, and go from there. I think this little carbine has potential

  4. Oddly enough, I patterned some 00 Buck today.

    I hadn’t done it in awhile, figured it was time to reconfirm the spread of the ol’ 12 gauge at various ranges with my HD ammo.

    • If you are measuring your groups in cubits you need a bit more practice before measuring accuracy is an issue.

      Alternatively, you could just pray to Yahweh for help hitting what you aim at, then group size is not an issue.

      • Since cubits are a rather imprecise measurement standard (from your elbow to your fingertip), there is no need to convert it. A good guess should be close enough in cubits.

  5. Join the nbrsa learn about benchrest shooting from the pros and get a cool magazine and learn the truth about precision shooting

    • Rick, I’ll kook in to that. I used to subscribe to Precision Shooting magazine, until they went under in Nov. 2012. Now I take Varmint Hunter and just received a book from my friend Mic McPherson. The name is “Metallic Cartridge Handloading” (2013).The author is Mic McPherson. I would urge anybody who handloads to get this book!

  6. Problem with calipers is they generally only go out to about 6 inches. I generally need a 25 ft. tape measure for my groups.

    Honestly, I think measuring shot groups with precision instruments officially qualifies you as a measurebator. I’ll change my mind if I ever see hickok45 do it.

    • Actually you should subtract .308 since that is the bullet diameter, if you use .303 or 7.62×54 you should subtract .310 or .311.

      Now with .38 special you have to subtract .357 since that is the bullet diameter.

      • Yes the whole post is actually rather misleading because bullet diameters are not always the actual caliber size in standard calibers (.38, .303, .32, .300, etc etc) and are pretty much never the measurement in metric calibers (7.62xWhatever, 5.56). The metric designations almost always refer to the bore diameter — the distance between the lands whereas the bullets are typically sized to fill the distance between the grooves. A 7.62×39 bullet is 0.312″ (7.92mm) in diameter while 7.62mm converted to inches is 0.300″. Our pal the .308 Winchester AKA 7.62×51 is actually a 0.308″ diameter bullet but in millimeters that’s 7.82.

        5.56 is actually 5.7mm in diameter.

        Again, in a metric caliber the designation is bore diameter, not bullet diameter. In standard it’s usually bullet diameter but it very often is not, as in the examples above. .303 Brit is .311 and .38 is .357, .44 is actually .429etc…

        Anyway 1) even if you eyeball center-to-center it won’t be any more off than assuming the diameter of a bullet when you’re wrong about it! and 2) measure inside edge of the hole on one side to outside edge of the hole on the other if you can’t eyeball the center accurately! This gives you the EXACT same measurement as center-to-center with no math and no eyeballing.

  7. Good post. The only groups I truly obsess over are those I shoot from my custom build .308 precision rifle. Here is a ten round group I was able to achieve the second time I took the rifle out, shooting from the shoulder with a bipod. The first time, during a precision rifle class, I was able to put rounds on a steel torso target, 11 x 14, at 740 yards.

    http://cyberbrethren.com/wp-content/uploads/2013/12/tenshotgroup1.jpg

    Otherwise, obsessing over groups from an AR or AK or other rifle never designed or intended to be a precision rifle can be an excercise in endless futility and the cause of spending 1000s of bucks to try to turn a good, functioning assault rifle into a precision rifle. To each his own though. The thrill is in the hunt for that super-tight group.

    In the real world if you can get rounds into an 8.5 x 10″ sheet of paper at 100 yards from your rifle you are good to go. Putting multiple rounds into the same basic wound channel on a human being is NOT what you want to do to begin with in a gun fight. The goal is to poke as many holes as possible, as quickly and accurately as possible, into a bad guy’s hydraulic and/or electrical system, creating as many wound channels as possible, causing the greatest possible and fastest loss of blood you can. Sorry to be so graphic about it.

    If you put holes in a target while moving and drilling and can cover them with your hand, that’s more than good enough.

    If you want really precise and accurate groups, just go for a precision rifle with a good scope, good trigger, etc.

    • Paul’s contribution here parallels my thoughts pretty well. Seriously I don’t know what’s more pathetic, a blog post about how to measure shot groups with precision instruments, or people arguing in the comments section about how to measure shot groups with precision instruments. Don’t forget that paper expands and contracts with moisture and humidity, so you’ll need to do that in a laboratory with a controlled atmosphere.

      Every forum should have a special section for measurebators. You’re more than welcome to borrow my digital calipers, but please pass me another box of ammo first. You can measure, record, put it all in a spreadsheet and do a regression analysis. I’ll be practicing, training, learning and having fun.

  8. I always measure outer edge to inner edge of the 2 furthest apart holes. Easy to see/feel compared to the empty space of center of hole.

    • Yep, no conversion or math necessary that way.

      Also, the way the article is written would have you believe that the bullet diameter of a .38spl is 0.38, or .44mag is 0.44. This is not true at all. ”Pro” tip indeed.

      • Is that difference truly significant? Most likely not. In fact, leaving sig figs to 0.1″ is prolly just fine given eyeballing the hole and tears.

    • I agree with your post. 3 shot groups are not the way to go, unless, you are shooting a new rifle, and are on a budget. There are so many variables involved when trying to find a load that will shoot respectfully in your gun, bullet weight, and style, powder type, and weight. primer selection, neck tension, and seating depth. I probably left out a few.

      When I shot my 16, 3 shot groups, I was just trying to get an idea of what might work, and what probably wouldn’t. If your three shot group, from a precision rifle is 1 1/2″ center to center, you know that it not worth the trouble to develop this particular combination any further. Two more shots in this group are not going to make it any better.

      If you get say, four or five good groups, then you can elaborate on these, and shoot 5 shot groups, for further development.

      Of course, if you have the time and money, then you can start right off shooting 5 shot groups, but for a lot of us, that is not always the case.

    • I dont shoot groups to check my accuracy. I shoot groups to check ammo and gun/barrel accuracy. I will take my 3 shot groups via my caldwell lead sled dft over the American Riflemans 5 shot groups via sandbags anyday.

  9. If I may point out the reasons for measuring groups. It can be to determine the accuracy of a given firearm with a certain load. If the accuracy of the load is known it can be used to see if your ability or the ability of the firearm has changed. It can be used to compare the accuracy of different ammo and different techniques in shooting styles.

    If you want to improve your shooting ability having a baseline for the ammo you are practicing with is necessary so you can gauge your improvement or lack of.

    In practical pistol shooting whether for competition or for self defense to think that speeding up or slowing down is a good way to adjust your groups to whatever size you think will do the job is folly. When groups open up it means you are less in control, we are responsible for every round that leaves our guns and should never sacrifice control for speed. Speed is a good thing but it comes from the skill of controlling your firearm at a higher rate of fire.

    The recent video of the policeman firing a 6 shot burst at an old man who reached for his walking cane is a graphic example. Six rounds fired with one hit. Unfortunately many think this is a good strategy, fire a bunch of bullets and one of them may hit, if not it will at least suppress them. Suppressive fire has no place in a civilian self defense situation and would be a damn rare LE situation.

  10. Paul needs to remember that semiautomatic rifles are sporting rifles not assault rifles. To his credit he called his modern bolt action rifle a precision rifle not a sniper rifle.
    I would call it a mulligan if he didn’t get the second one right.
    Classifying a sporting rifle as not accurate or capable of precision is inaccurate. Many many people have sporting rifles that are sub moan guns, by design or by the quality of the manufacturers design and parts.
    That doesn’t always equate to cubic yards of cash.

  11. I would also add that the calipers should be held (unlike the lead-in pic) parallel to the line between the two farthest bullet holes, and not level with the ground – otherwise you are cheating, hehe.

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