Measuring Sub-Caliber Group Size

[dnttech]

Hi...As a new member here, I have a question for those who may be able to help. I recently shot a 100 yard, 3-round group from the bench where the resulting hole measured smaller then the actual diameter of the bullet.

In this case, exactly how is the group measured? Is it simply a matter of measuring the hole edge to edge? If this is the case, the hole measured .288 while the bullet measured .308. Would this then be measured as a .288 inch group? I obviously can't subtract the diameter of the bullet since this would give me a negative number.

While I've been able to shoot quite a number of groups in the .00x range, this is actually my first time shooting a group smaller then the diameter of the bullet.

I've asked quite a number of my fellow shooters how to approach this, and quite simply, no one was sure.

Thank you in advance for any help you may provide.

P.S. By the way, for those who may wonder why this isn't a 5-round group, I just didn't load up enough rounds. This was a test load I was working up for this particular rifle.

P.S.S. I've tried loading a picture of the actual target, but apparently the size of the image is too great.

[Ske1etor]

http://www.ontargetshooting.com/download1.html

This is how.

With a ruler or caliper, measure from outside of grease ring to outside of grease ring and subtract bullet diameter. If your single hole is inside the bullet diameter, the spread is 0.

[JohnKSa]

...the resulting hole measured smaller then the actual diameter of the bullet.

It's not unusual for a bullet hole in a paper target to be smaller than the bullet diameter itself--sometimes it is significantly smaller. This is why I take every opportunity to decry the method of measuring edge-to-edge of the outside shots and subtracting the bullet diameter. That method almost always underestimates the group size.

As your group proves, it's even possible to shoot a group and end up with the overall size of the hole in the target being less than the bullet diameter. Your overall group size is more than 6% smaller than the diameter of the bullet you were using.

One option would be to always shoot a few test shots outside the group to characterize the size of the hole that the bullets make in the paper. Carefully measure the holes, average their sizes and use that average number to subtract from the edge-to-edge group size rather than simply using the nominal bullet diameter.

[dnttech]

Ske1etor and JohnKSa, thank you both for your input.

Ske1etor, I used exactly your method for determining the hole size...outside of grease ring to outside of grease ring, with a caliper, and came up with the .288 measurement.

While my normal bench rifle is a 6br, I'm happy I was able to work up a load with which the rifle was happy. I'm just getting my son into bench shooting, and the .308 he's using with the round I worked up should suffice for the time being.

Again, my thanks...

Bob

P.S. JohnKSa, my apology for neglecting to mention your input. Your suggestion of shooting several shots (I'll do 5 shots) and determining the average of the shots makes a lot of sense. For the number of years I've been shooting, I can't believe I didn't think of that. Thank you....

[Bart B.]

That bullet hole edge on target paper is typically called the "leaded edge." It comes from competition shooting over a century ago when lead bullets were the only thing around. It now also applies to jacketed bullets.

One thing about measuring groups is the odds of which those of different sizes occur when a few to several shots are in each group. All fired groups with 2 or more shots in them will be somewhere between near zero units of measurement and some larger number. A few will be very small, several around average and a few very large. Smaller ones happen when all the variables tend to cancel each other out in all directions. The largest ones area the result of all the variables adding up in all directions. The biggest variables are that of the shooter with lesser ones caused by the ammo and rifle.

I suggest one concentrate on getting the largest groups smaller. Ignore the smallest ones shot because it's impossible to tell if everything was near perfect or just variable-cancelling luck; the latter's more common than the former. What's more accurate, load A that shoots 3-shot groups at 2, 4, 6 and 8 units or load B at 3, 4, 5 and 6 units? Any load will shoot a very tiny group once in a while.

[skizzums]

I don't know because I don' find myself having your problem, but congratulations. time for you to start competing

[Jimro]

If you know the bullet hole for a single bullet is .XX inches, measure extreme spread for the group and subtract the diameter of the single hole from the group extreme spread.

Another option is to use a target system with automatic scolling witness paper behind the aiming target.

I think the easiest method is to use a scanner and target analysis software as ske1etor suggested.

Jimro

[dnttech]

Hi..

I am finally able to get back online and update information concerning the questions I posted about measuring sub-caliber group size.

I fired 5-shots and used the average diameter of the individual holes to come up with a .272 size. Strangely, the holes all measured quite a bit different in individual sizes. I wasn't quite expecting the disparity...nonetheless, .272 is the average.

Using this as the baseline, I was able to come up with a 3-shot group size of .016. In this case. .288 inches measured leaded edge to leaded edge, minus the .272 average of the control group. Thank you Bart B. for clarifying that piece of vernacular.

I loaded 25 rounds of ammunition with the intent of using 5 of the shots for my comparison group and the remaining 20 rounds to verify the inherent accuracy of this particular load.

I won't bore you with the information I note when it comes to atmosphere, elevation and all the other pertinent data.

For this particular rifle, my first 5-shot group measured .203, the second 5-shot group measured .212. I then gave the range officer, good friend and excellent rifleman, the bench. His 5-shot group measured .297. My last 5-shots were my best of the day and measured .183. Suffice to say, I'm happy with this load.

Bart B. Thank you very much for your input on this matter as well. When I looked at the credentials you have posted, I feel it's quite an honor that someone with your experience would be kind enough to give me the benefit of your knowledge.

[Bart B.]

dnttech, you're welcome.

Note that in some country's ways of measuring shot groups, they use the furthest shot hole edges and don't subtract bullet diameter. I learned this years ago asking Eley (in England) about their 22 rimfire match ammo accuracy. Their rep said they'll never get a group less than 5.69mm (.224"), the diameter of fired rimfire bullets.

[dnttech]

Hi Bart...

Personally, I like the idea of measuring furthest edge to furthest edge. Makes sense to me. I've wondered in the past why we don't measure in that manner.

I'll be turning this rifle over to my son, shortly...I'm positive it can repeat a sub-caliber group size with this load. I purchased a NightForce Benchrest scope to go on this rifle, so after mounting and sighting it in for him, it should be ready.

I've found a new challenge in .22 rimfire that I'm trying to perfect. After extensively modifying a CZ455, I think I can get my groups into the .15 to .20 range at 100 yards. if I can do that, I'll be very happy. Right now she's just breaking into the high .2's consistently. Damn Lapua ammo is hard to find, though...and getting expensive.

[Bart B.]

The reason that group measurements in the USA have been traditionally made between bullet centers furthest apart instead of outside edges is so two or more different calibers can be realistically compared.

A group with a 25 caliber rifle whose outside edges measure 1 inch apart will have those two bullet centers .75" apart.

A group with a 30 caliber rifle whose outside edges measure 1 inch apart will have those two bullet centers .70" apart.

A group with a 35 caliber rifle whose outside edges measure 1 inch apart will have those two bullet centers .65" apart.

Which caliber rifle's group is the most accurate?

What would the outside edge measurements be for each caliber if all three measured exactly 1 inch between extreme shot hole centers and therefore had equal accuracy (or precision, if you prefer)?

Benchrest competition at short range typically uses 22 or 24 caliber bullets. The targets are scored based on center to center of widest shots. Otherwise, if outside edges were used, no 24 caliber bullet would have a chance of getting a "smaller group". The playing field has to be leveled.

In NRA high power rifle matches, the scoring rules used to state that a 30 caliber scoring plug (.308" measuring flange diameter) would be used for all calibers. That way, any caliber bullet hole whose center was .153" away from a scoring ring would be given the higher value as the flange would be into the scoring ring edge. But that got changed and now the scoring plugs are caliber specific; the advantage is given to people shooting 30 caliber bullets. A 30 caliber bullet hole whose center is .1535" or less off the 10-ring will be scored a ten. A 22 caliber bullet hole's center that's .1535" off the 10-ring will be scored a nine because its scoring plug flange diameter is .224" and has to be no more than .112" away from the scoring ring to get the higher value. Check out Rule 14.3 How to Score on page 45 in:

http://compete.nra.org/documents/pdf...R/hpr-book.pdf

Best wishes getting any .22 rimfire ammo to consistantly shoot under 1/2 inch/MOA at 100 yards. The best match ammo made before the mid 1980's would easily do that. But after an explosion at a rimfire ammo plant's primer mix site then, primer chemistry was changed by all companies. Ammo's never been that good since; one exception was when a Russian ammo plant made some in the early 1990's that was imported into the USA for a few years. Virtually all the 100 yard records shot before the 1980's still stand. The best that available today will shoot inside 3/4 inch/MOA at 100. And it wears out barrels faster, too.

[dnttech]

Hi Bart:

Thank you for the information in reference to scoring. I understand, now, the concept in measuring center-to-center as opposed to measuring leaded edge to leaded edge.

The CZ455 I've recently begun shooting can "always" shoot within the aforementioned 3/4-1/2" MOA at 100 yards...with the correct ammo and with me doing my part. It's taken a bit of money to get it into the high .2's, low .3's, but even at that, I can't do it every time. If I find myself shooting outside 1/2", the rifle goes back in the case and I simply wait for another day. My normal shooting time for the .22 is very early morning or late afternoon...

My biggest gripe is the flyers I get on occasion. They totally wreck my group. I'm unable to tell if it's the shooter, the gun, the ammo, atmosphere or a combination of all the above.

I've used Wolf, Eley, SK, Lapua..finally settled on Center X as the most consistent...but even at that, I still get an occasional flyer.

I've had a number of people tell me to go to RimfireCentral and find out about matches, world records, etc., but that doesn't interest me. I'm only looking to shoot the very best I can with the best equipment I can afford.

According to one of the range officers who shoots with me, my personal best 10 shot group at 1000 yards is better then the official world record. That was done with a box stock Sako TRG-42, NightForce scope and my own load. To me, it's like...so what? I have no interest in records or how well other people shoot...I just love to shoot and shoot to the best of my ability.

The thing that interests me the most is scoring my targets and measuring group size correctly. The reason for my initial post. I've shot a great many handgun matches over the years and I suppose I had a modicum of success. but never ran into an issue like my original question.

[Bart B.]

There are no fliers. Every shot counts unless you call it where it hits. Fliers are those few shots reflecting all the variables adding up in the direction they go.

[James K]

The size of a bullet hole cannot be smaller than the bullet that made it. But, due to the flexibility of paper, the hole can "close in" so it looks smaller than the bullet, which is why scorers (those folks who mark and score targets) use gauges of bullet diameter to tell exactly where the hole is.

It is also the reason pistol shooters like to use "wad cutter" bullets, which cut a clean hole in target paper and make scoring easier. (The name comes from the appearance of paper from which shotgun wads have been cut for reloading shotshells before the days of plastic shot cups.)

But it is perfectly possible, even common, for a GROUP SIZE to be smaller than the diameter of the bullets that made it, since it is the center-to-center distance of the farthest holes.

One of the ways to eliminate that closed in look in bullet holes is to use a different target paper. Standard target paper (as opposed to, say, printer paper) will be better; stiff cardboard or corrugated box board may be better.

Jim

[Bart B.]

dnttech,

Note that the real accuracy any rifle can be counted on to shoot all the time is somewhere between zero and the largest group fired. Too many people shoot lots of loads trying to get the smallest group possible then expect them to all be that tiny. Any load will shoot almost zero once in a great while if you shoot enough groups, but you may well wear out the barrel or your bank account paying for it. You've admitted that by saying "It's taken a bit of money to get it into the high .2's, low .3's, but even at that, I can't do it every time."

When one shoots several few-shot groups and they're not all the same size within 10%, none of them have enough shots to represent the accuracy level that can be counted on all the time. The real accuracy level of .22 rimfire rifles doesn't change over a few thousand shots. After 20,000 shots, good marksmen will see a slight degradation. By 30,000 or a little more shots, accuracy will have degraded too much for serious competition so the barrel's replaced.

Remember the smallest groups happen when all the variables cancel each other out in all directions; that doesn't happen very often. Biggest groups happen when all those variables add up. Use the load where they add up the least amount. For rimfire, shoot a 30- or 40-shot group of different makes and types. Use the one that produces the smallest group of that many shots.

I don't think there's any .22 rimfire ammo made these days that'll shoot under 1/2" at 100 yards all the time. But any of it will put a few shots into 2/10" once in a (great) while. 3/4" is about as good as anyone gets these days.

[SG29736]

"I've had a number of people tell me to go to RimfireCentral and find out about matches, world records, etc., but that doesn't interest me. I'm only looking to shoot the very best I can with the best equipment I can afford.

According to one of the range officers who shoots with me, my personal best 10 shot group at 1000 yards is better then the official world record. That was done with a box stock Sako TRG-42, NightForce scope and my own load. To me, it's like...so what? I have no interest in records or how well other people shoot...I just love to shoot and shoot to the best of my ability.

The thing that interests me the most is scoring my targets and measuring group size correctly. The reason for my initial post. I've shot a great many handgun matches over the years and I suppose I had a modicum of success. but never "



Oh, come on. Jeez. If you are only interested in shooting your best and not comparing it to other shooters, what's the difference how you measure it. Pick a method and you can still measure your own improvement compared to your past results. Or go to some matches and see how you do in those conditions. Mark

[Bart B.]

The smallest few-shot groups for a given load fired with rifles held against ones shoulder shot at any range, realistically, are often about 5% rifle, 5% ammo, 10% shooter and 80% luck.

[dnttech]

Sorry, I've been out of town and didn't get a chance to view any of the newer posts.

Thanks to everyone who responded to my initial question.

Mark, your post really sums up my initial question when you mentioned, "you can still measure your own improvement compared to your past results." I guess that's really all I'm trying to do.

In all sincerity, I have no interest in matches. I've shot hundreds of handgun matches in my 33 years as a police officer, but my foray into rifle shooting only began about 4 years ago after I retired. I suppose I simply became burnt-out when it came to matches and have no desire to shoot in that environment again.

What I love about rifle shooting is the fact it's still a whole new animal to me and I learn something new each time I go out....usually morning and late afternoon. Shooting the .22's at 100 yards is sort of a sub-set of that, mixed with a ton of frustration....

Bart, your response about 80 percent luck is about right. I've been told about a Lapua product I haven't tried yet...I believe it's called Midas. If that shoots better then Center X, maybe I can reduce the 80 percent somewhat.