Measuring Group Size
- Thread starter Taylorman
- Start date
P1ZombieKiller
Silver $$ Contributor
Center to Center (glad you didn't ask if it should be 3 shot or 5 shot group.... you would have started another fight that I'm just too tired to get into) 
Easiest way to get group size is measure the farthest point from outside to outside, and subtract your bullet diam.
Example: group measured .550 and you shoot a .308 then your group is .242
Easiest way to get group size is measure the farthest point from outside to outside, and subtract your bullet diam.
Example: group measured .550 and you shoot a .308 then your group is .242
Yes to what Mike said. The rifle manufacturers, plus the magazine gun writers, when they review a "new" rifle, showing one 3 shot target fired at 100 yds., and the guy who carries a "group" in his wallet, who can't wait to show it claiming "this is what my rifle will do". Completely meaningless. Show me what it will average with multiple ( the more the better) 5 shot groups at 200 yd. Recently saw some advertising crap from a smallbore rifle manufacturer, showing a 3 shot "group" fired at 25 feet. 
AlNyhus
Silver $$ Contributor
The correct way is: measure outside-to-outside (include the black 'smudge line around the bullet holes), then subtract the diameter your bullet makes in the paper.
For example, if you're shooting a .30 and the dia. of a single bullet hole measures .290, you would subtract .290 rather than .308 from you outside-to-outside figure.
Like this:
For example, if you're shooting a .30 and the dia. of a single bullet hole measures .290, you would subtract .290 rather than .308 from you outside-to-outside figure.
Like this:
I dont shoot in competition, but are you telling me this is how they measure group size?
ok lets talk about a 308 bullet, ill give you this, it might be only a .307 after traversing the barrel, but its not .290 when it gets to the target, so what happened to the other .017" if its not paper spring back.
If, with your paper, all 5 308 holes spring back to .290, and thats what you use to get the OA group spread, then that would be the correct number to subtract.
However if you used a paper target that a 308 produced a 307 hole, would you not subtract .307?
Some target paper is terrible about spring back, and others are not, printer paper may be the worst for this.
Is this your justification for using .290 as the deductible?
I just dont want people to think a 308 bullet makes a .290 hole, rather only when it hits certain types of paper, and after it springs back. When that bullet passed through the paper,(any paper) it made a .307 to .308 hole, (assuming its not key holing) correct?.
So to measure correctly, it would depend a lot on the paper used.
Mike.
ok lets talk about a 308 bullet, ill give you this, it might be only a .307 after traversing the barrel, but its not .290 when it gets to the target, so what happened to the other .017" if its not paper spring back.
If, with your paper, all 5 308 holes spring back to .290, and thats what you use to get the OA group spread, then that would be the correct number to subtract.
However if you used a paper target that a 308 produced a 307 hole, would you not subtract .307?
Some target paper is terrible about spring back, and others are not, printer paper may be the worst for this.
Is this your justification for using .290 as the deductible?
I just dont want people to think a 308 bullet makes a .290 hole, rather only when it hits certain types of paper, and after it springs back. When that bullet passed through the paper,(any paper) it made a .307 to .308 hole, (assuming its not key holing) correct?.
So to measure correctly, it would depend a lot on the paper used.
Mike.
Mike,
I don't shoot in competion either. I have seen a caliper used that looked to me like it had a stencil or something on it they used to very accurately locate the bullet hole on paper.
Let me take a stab at this:
If you are measuring with a caliper, tape measure, leatherman, whatever, it's important to keep your measuring constant. It shouldn't matter if your 308 for some reason makes a hole that is .500" round in your paper. Measure outside to outside 'on the paper' and subtract the diameter of the hole your bullet is making 'on the paper'.
For whatever reasons holes in paper are usually smaller. If that's the case and you measure outside to outside 'on paper' and subtract bullet 'actual' diameter your group sizes will be smaller than they really are.
Try the free program listed above it's pretty slick. Attached is a group. When you use the program you input your bullet actual diameter then a circle appears on the screen and you drag it over the holes. You need to eyeball it as best you can. It's pretty neat. Calculates some other stuff. Gives you a neat file to send to your buddies that everybody can read. My apologies for the three shot group. I was just looking for a 300 yard zero (15 degrees, snow, strong wind miserable).
Jeff
I don't shoot in competion either. I have seen a caliper used that looked to me like it had a stencil or something on it they used to very accurately locate the bullet hole on paper.
Let me take a stab at this:
If you are measuring with a caliper, tape measure, leatherman, whatever, it's important to keep your measuring constant. It shouldn't matter if your 308 for some reason makes a hole that is .500" round in your paper. Measure outside to outside 'on the paper' and subtract the diameter of the hole your bullet is making 'on the paper'.
For whatever reasons holes in paper are usually smaller. If that's the case and you measure outside to outside 'on paper' and subtract bullet 'actual' diameter your group sizes will be smaller than they really are.
Try the free program listed above it's pretty slick. Attached is a group. When you use the program you input your bullet actual diameter then a circle appears on the screen and you drag it over the holes. You need to eyeball it as best you can. It's pretty neat. Calculates some other stuff. Gives you a neat file to send to your buddies that everybody can read. My apologies for the three shot group. I was just looking for a 300 yard zero (15 degrees, snow, strong wind miserable).
Jeff
Attachments
Al, Thats what I called DIALED IN!!! Very Nice.
Pardon my ignorance but do you sell those 118's?
I'm thinking I need a gun with a bigger hole in the end of it this year.
My 2 cents
Internet groups = Group size - bullet caliber.
Competition = Als method.
Its usually two different worlds ain't it.
Pardon my ignorance but do you sell those 118's?
I'm thinking I need a gun with a bigger hole in the end of it this year.
My 2 cents
Internet groups = Group size - bullet caliber.
Competition = Als method.
Its usually two different worlds ain't it.
I'm not sure how they work, but I have a picture of a 3 round test group target fired at 100 yds in an underground test range. The target must be projected on to a screen, it then puts cross lines (vertical and horizontal) over the center of each hole, then calculates the Center to center of the furthest apart two of the three or five, and gives you the group size.
This is not something you would find at some guys house. Rather it was at a quality rifle makers facility.
Jeff, ill check that out, it sound sort of like the test target I saw used by a manufacture. (see above) I have also taken unfired rounds and place them in bullet holes, and with a turn or two you can push it down until the case neck touches the paper, even mark the outside if you want, but in this case you would deduct the actual bullet size (ie 308), It does get a little more complicated with a raged one holer.
I have a lead bullet mold for a 41 Mag pistol, and its a semi wad cutter bullet for target shooting, and has a sharp rim around the outside, and it makes holes that look like you used a paper punch, no problem in reading these holes.
And Jo1911, I think your right. And Jeff, you never said anything about 3 hole groups, but some newbies do.
Mike.
This is not something you would find at some guys house. Rather it was at a quality rifle makers facility.
Jeff, ill check that out, it sound sort of like the test target I saw used by a manufacture. (see above) I have also taken unfired rounds and place them in bullet holes, and with a turn or two you can push it down until the case neck touches the paper, even mark the outside if you want, but in this case you would deduct the actual bullet size (ie 308), It does get a little more complicated with a raged one holer.
I have a lead bullet mold for a 41 Mag pistol, and its a semi wad cutter bullet for target shooting, and has a sharp rim around the outside, and it makes holes that look like you used a paper punch, no problem in reading these holes.
And Jo1911, I think your right. And Jeff, you never said anything about 3 hole groups, but some newbies do.
Mike.
AlNyhus
Silver $$ Contributor
Mikem: Bullets make different sizes in target paper due to the flexibility of the paper. You're correct that different types of target paper will produce different size holes with the same bullet diameter.....that's why you use the diameter of a single hole in the target paper being used as the 'standard' when figuring group size. Here's a target shot with a .25...as you can see by the figures, a single bullet hole in this instance made a .240 hole in the paper, so that's what I used to figure group size:
In IBS/NBRSA sanctioned BR events, a measuring device is attached to a caliper and scribed outside circles that coorespond to different bullet diameters are lined up on the outside of the holes to get the group measurement.
Charlie and Cheryl Hood make the slick little Kwik Estimator that uses tapered lines to achieve the same result. http://www.benchrest.com/hoodpress/kwik_estimator.html
Good shootin'. -Al
In IBS/NBRSA sanctioned BR events, a measuring device is attached to a caliper and scribed outside circles that coorespond to different bullet diameters are lined up on the outside of the holes to get the group measurement.
Charlie and Cheryl Hood make the slick little Kwik Estimator that uses tapered lines to achieve the same result. http://www.benchrest.com/hoodpress/kwik_estimator.html
Good shootin'.
-AlAlNyhus
Silver $$ Contributor
jo191145: Thanks for the kind words. Randy Robinett of BIB Bullets is a good pal and my bullet making mentor. Any success I've had with my bullets is ultimately a result of his willingness to share his wealth of knowledge with me.
I don't make bullets for sale commercially, but would highly recommend Randy's bullets to you. His .30's are the Gold Standard to which all other BR grade .30's are compared. 8)
Good shootin'. -Al
I don't make bullets for sale commercially, but would highly recommend Randy's bullets to you. His .30's are the Gold Standard to which all other BR grade .30's are compared. 8)
Good shootin'.
-AlNorCalMikie
Gold $$ Contributor
NorCalMikie
Gold $$ Contributor
You "really" know how to hurt a guy. That's elder abuse! 
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