How does one measure group size?

[cropcirclewalker]

I guess one bullet hole has a group size of zero.

I guess two bullet holes have a group size of the center to center distance between them.

I guess three+ bullet holes have a group size of the maximum center to center distance between the two holes that are farthest apart.

Right?

 

[Zak Smith]

Yes.

 

[cropcirclewalker]

Thank Yew.

 

[Brian Williams]

I find the 2 hole that are the farthest apart and draw a line between the 2 and measure from the edge of one hole to the same edge of the other, so I include the distance between the 2 holes and the diameter of one hole. It is hard to find the center of a hole but not the edge.

 

[cropcirclewalker]

Mr. Williams, I like your news better than that BrokeJaw guy, and thanks for the suggestion.

I am building a spreadsheet to figure the group size, so I need the x,y cooridinates of the center of the holes, which I read off a target which I created with 1/8" grids printed on it.

I intend to use the old ((X1-X2)^2 + (Y1-Y2)^2)^.5 (pythagorically speaking) to each pair of holes, then apply a macro to select the largest.

That's why I was asking.

 

[Charles S]

intend to use the old ((X1-X2)^2 + (Y1-Y2)^2)^.5 (pythagorically speaking) to each pair of holes, then apply a macro to select the largest.

Insert Mr. Burns voice (of the Simpsons).

Excellent.

Charles

 

[JackOfAllTradesMasterAtNone]

If my group is outside the 9 ring on any target designed for any distance, then it's a BIG group. No need to measure. If my group is well within the 9 ring, then it's something to brag about to most folks. Still no need to measure.

(Two farthest holes, Outside edge of one hole to the outside edge of the other.) I can cover my shots with a dime, nickle, quarter...

-Steve

 

[cropcirclewalker]

So, Mr. Jack......

When I am working up a load and tweaking it I should just record the group size in my book as;

Pizza Pan
Dinner Plate
Saucer
Soda Can
Shot Glass (top)
Shot Glass (bottom)
Half
Quarter
Nickel
Penny
Dime
20 gauge
.45
.44
.38
.303
.30
.25
.22
.17

I guess there is no need to go smaller than that.

 

[JackOfAllTradesMasterAtNone]

Depending on what you're shooting, at what distance, your analogy of group size might be commendable. Say, a 5" something or other mounted on a really big boat, with groups the size of a truck at say, four miles, then I guess that's a pretty good group.

Seriously though. I put a ruler across the holes that are the farthest apart. Outside edge to outside edge. That's what I record in my shooting log. It's too difficult to determine center of the hole. Especially if all of them over lap each other.

-Steve

 

[Smokey Joe]

Center to center is easy with a shortcut!

For group size furthest center to furthest center, measure furthest outside edge to furthest outside edge, then subtract one caliber.

For example: I have a nice group in front of me now. Put the calipers on it and it is 1.044" outside edge to outside edge. Minus 0.308 = 0.736" group size.

Which, while it ain't benchrest accurate, ain't too bad for 100 yds with the load only halfway developed for that rifle, case, bullet, and powder.

BTW, I don't brag about a 0.736" group. I just say it's under 3/4".

MUCH easier than trying to figure out where the centers of the holes are!

 

[Zak Smith]

Yep, I measure the outside of the two furthest holes with a caliper (handy, easy to read) and then subtract one bullet diameter.

Note that if you measure and record max furthest edge (without subtracting caliber) you can never get a group smaller than the bullet diameter, which is annoying in 50BMG!

-z

 

[Charles S]

Yep, I measure the outside of the two furthest holes with a caliper (handy, easy to read) and then subtract one bullet diameter.

I find that this is the easiest way to measure group size.

Charles

 

[JackOfAllTradesMasterAtNone]

Isnt' that the point? You can't get a group smaller than bullet diameter! Doesn't that explain the phrase, "One ragged hole". When you can use that phrase, then "group size" doesn't matter. :banghead:

OK, yes, measuring outside edge to outside edge, then subtracting one bullet diameter will get you center of the holes.. I just don't do it.

-Steve

 

[cropcirclewalker]

Many times I have the "One Ragged Hole" problem, herinafter referred to as ORH, which is why, when working up smally grouping loads, I like to cover each bullet hole with a garage sale sticker before I make the next shot.

Nothing so disconcerting as firing 5 and then seeing only ORH. Makes it hard to work up definitive loads. Takes a lot of time too when I have to keep looking down the barrell to insure that I don't have a lodged one.

Each shot gets it's x,y coordinates logged in along with velocity and then I let the computer do the figgerin'.

Not only that, but let's say I shoot 5 and see 4 in ORH with a flyer an inch away. Is that the load? Or the loader? Or the shooter? What?

Duplicate and try again. That's what I say.

 

[Zak Smith]

Then you can't meaningfully compare the accuracy of different calibers, because a 30-caliber rifle could shoot no better than .30", while a 50BMG could shoot no better than 0.51"

 

[obm]

to get the center to center measure you take the measurement from the outside edge of the one of the furthest holes and measure to the inside edge of the other furthest hole. that will give to c-to-c.

 

[WayneConrad]

Nothing so disconcerting as firing 5 and then seeing only ORH.

Oh, to be thusly disconcerted!

 

[Smokey Joe]

Slight problem there!

OBM--You are quite correct in your observation, sir, but may I point out that there is an IF. While your method is theoretically just as good as measuring outside to outside, and avoids the subtraction, I submit that it has a problem, to wit: If you are dealing, as is ideal, with one ragged hole, the inside edge of any one bullet hole will be a little difficult to find.

If the group is spread enough that each bullet makes a separate hole in the target, then your method works, and is more efficient.

The outside edge of all the outer bullet holes, however, are always right there. That is why most persons willingly measure outside to outside edge, then subtract one caliber.

 

[obm]

OBM--You are quite correct in your observation, sir, but may I point out that there is an IF. While your method is theoretically just as good as measuring outside to outside, and avoids the subtraction, I submit that it has a problem, to wit: If you are dealing, as is ideal, with one ragged hole, the inside edge of any one bullet hole will be a little difficult to find.

but there's ALWAYS a flier.

(point well taken, sir)

 

[milanuk]

Oh, c'mon now. To be fair you need to shoot one shot, measure the actual size of the bullet hole in the paper (the paper stretches slightly as the bullet passes thru, so the hole is actually slightly less than bullet caliber), and subtract that from any further groups sizes instead of nominal bullet diameter...

If you need to do that, you are shooting a whole lot better than I am!

Monte

 

[JackOfAllTradesMasterAtNone]

"Oh, c'mon now. To be fair you need to shoot one shot, measure" "and subtract that from any further groups sizes"

You've got me confused. One shot does not make a 'group'.

-Steve

 

[Oldnamvet]

HOw 'bout just getting one-in-a-row and calling it good?

 

[milanuk]

Never said one shot made a group. Shoot one shot off to the side, call it your 'cold bore zero' shot or whatever trips your trigger. Measure it, and use the actual size of the hole in that kind/type/weight of paper when subtracting bullet diameter from any subsequent groups shot on that kind of target. Really a matter of picking nits, unless you are shooting bona fide 'one holers' three or five shot groups, and really care about the difference (some people do).

YMMV,

Monte

 

[OneInchGroup]

Determining Group Size

Coming in to the discussion a bit late, but it seems there is some confusion about group size measurement here. Our games are mostly all based on 5 shot groups fired at 50 or 100 yards. This does not lend itself especially well to caliper measuements since nobody's calipers can check 5 axes at once and make any sense of it.

The official method used in our Nationals uses a graduated ring gauge, basically a set of scribed circles on a clear acetate sheet. The ring gauge is set on the target, and moved around to include (cover) the outer edges of all the holes within a ring of whatever diameter it takes, and then the judges measure a single hole or figure in the diameter of the slug reported to have been used, and deduct that from the size of the "group ring".

What you have left is the group size, adjusted for the size of the slug used.

There is no point to judging any competition based on the size of a plate needed to "cover" all the shots, because then nobody firing a large bore gun could ever compete and win against somebody firing a .22, even if the big bore guy was a better shot.

Any more than 2 rounds, you have to use a circular gauge, and to mean anything in accepted competition "group size" has to refer to center distance among the shots measured, not some nonsense like "covered all the holes with my thumb" or whatever.

Everybody is welcome to agree, argue, P&M all they want, but if you are going to play in any of the serious benchrest competitions, whether its ours (American SlugShooting Association, www.slugshooting.com) or NRA, or ISP, or any of the rest, all are judged the same way.

An exception is when the contest is limited to a single caliber, where you can rank shooters according to what "covers" their group of shots.

To be meaningful outside that single contest, the actual score should still deduct the bullet diameter if the scores are to be compared to other contests with other size projectiles.

After all, the Ruger 22 guy with a 5 shot group covered by a 3" disk at 100 yards should not be seen as a better shooter than a guy shooting a 12 gauge slug gun at the same target whose shots are "Covered" by a 3 1/2" disk, and actually is grouping tighter than Mr. 22, once the size of the bullets is factored in.

Of course everyone is welcome to enter our contests and find out how good they really are, when the results get judged correctly.............

 

[cropcirclewalker]

Thanks

So, I got on my cad system and I drew a circle with a diameter of .308 with the coordinates of 0, 1.

Next I created a circlular array around the origin with 3 entities equally spaced, namely 120 degrees apart. Which is about as far apart as one could get.

So per the REAL way to measure the group, namely a circle which covers all the centers has a diameter of 2 inches. A 2.0 inch group as it were.

The center to center dimension of each hole to the others was 1.7321 inches.

Finally, if I array the holes with 4 of them at 360 degrees, the REAL group size is actually the distance between the centers of the most distant holes.

In conclusion it appears to me that one should never shoot groups larger than nor smaller than 4. Solves the problem.

Otherwise, does anybody know a simple algorithm, by which I can compute the actual group size of 3 shots on a spread sheet?