What is the correct way to measure groups on a target? For example, if you shoot 10 shots at a target, and your two shots farthest from the center of the bullseye are 3" left of center and 3" right of center, does this constitute a 3" group, since the two shots are no further than 3 inches from the center? Or would that be a 6" group, since the distance between these two shots is six inches? What is the correct method of measurement? Thanks for any assistance.
How do you correctly measure groups?
- Thread starter RM
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1911Tuner
Moderator Emeritus
Groups
Measure from the outside edges of the two that are farthest apart and subtract a bullet diameter.
This only gives credit to the weapon for its worst two shots, and none to
any that were closer. Useful in telling you that with a given zero, that the farthest from the point of aim that any shot will strike is half the group size...(2-inch group...the greatest distance from point of aim will be one inch.) This is assuming that the shot at the target is taken under the same conditions as the test group.
A better test of accuracy is to determine the center of the group, measure the distance of each shot from that point...and calculate the average.
Precise triangulation of the group center is a little complex, but the
"Field Expedient" method is nearly as good, and much easier. It's not
as precise, but it will do.
Using a straightedge, draw a square with its lines on the tangent of the
outer bullet holes. Keep the square as close to having 90 degree sides as possible. When the square is established, use the straightedge to form an "X" by drawing two intersecting lines from corner to corner. The center of the X will be the center of the group. It may not agree with the point of aim.
From that center, use the straightedge to determine the angle of departure
from the centerpoint to the center of the bullet hole... measure the distance from the center of the X to the outer edge of the hole on that angle, and subtract one-half a bullet diameter.
Do that for each hole, and find the average distance. That figure will
determine the average deflection from point of aim that you can expect with any given shot. Doubling the figure will determine the "true" group
size...or the worst that you can expect from the rifle on a given shot.
The "true" group size is a better indicator because it gives credit for all
shots, rather than only the two worst ones.
To test your own accuracy with the rifle, repeat the test by firing a 7-shot group from a field position...Your choice of standing, sitting, or prone...or
all three if you have the time and the ammo to burn. This will tell you what
the rifle/ammo combination is capable of in your hands under realistic
conditions. Be prepared for an humbling result. If your field group is three times the size of your benchrest group, you're an exceptionally good shot, and well above average. If it's only twice the size, you are sniper class. If the two are the same, you're one in about 10,000...a Master Class marksman.
Luck!
Tuner
Measure from the outside edges of the two that are farthest apart and subtract a bullet diameter.
This only gives credit to the weapon for its worst two shots, and none to
any that were closer. Useful in telling you that with a given zero, that the farthest from the point of aim that any shot will strike is half the group size...(2-inch group...the greatest distance from point of aim will be one inch.) This is assuming that the shot at the target is taken under the same conditions as the test group.
A better test of accuracy is to determine the center of the group, measure the distance of each shot from that point...and calculate the average.
Precise triangulation of the group center is a little complex, but the
"Field Expedient" method is nearly as good, and much easier. It's not
as precise, but it will do.
Using a straightedge, draw a square with its lines on the tangent of the
outer bullet holes. Keep the square as close to having 90 degree sides as possible. When the square is established, use the straightedge to form an "X" by drawing two intersecting lines from corner to corner. The center of the X will be the center of the group. It may not agree with the point of aim.
From that center, use the straightedge to determine the angle of departure
from the centerpoint to the center of the bullet hole... measure the distance from the center of the X to the outer edge of the hole on that angle, and subtract one-half a bullet diameter.
Do that for each hole, and find the average distance. That figure will
determine the average deflection from point of aim that you can expect with any given shot. Doubling the figure will determine the "true" group
size...or the worst that you can expect from the rifle on a given shot.
The "true" group size is a better indicator because it gives credit for all
shots, rather than only the two worst ones.
To test your own accuracy with the rifle, repeat the test by firing a 7-shot group from a field position...Your choice of standing, sitting, or prone...or
all three if you have the time and the ammo to burn. This will tell you what
the rifle/ammo combination is capable of in your hands under realistic
conditions. Be prepared for an humbling result. If your field group is three times the size of your benchrest group, you're an exceptionally good shot, and well above average. If it's only twice the size, you are sniper class. If the two are the same, you're one in about 10,000...a Master Class marksman.
Luck!
Tuner
Tuner is right. However the standard method for internet commandoes is to multiply actual yardage by 3 and measure from INSIDE EDGE to INSIDE EDGE and then subtract bullet diameter. It is theoretically possible to get a negative group size. But, most internet commandoes arent shooting that close together anyways even at 5 (15!!!) yards.
I have actually seen people use this method in the past and its worth a chuckle everytime.
I have actually seen people use this method in the past and its worth a chuckle everytime.
Thanks very much for your replies. What if your group of 7 shots are all within 2 inches of each other, but a full 6 inches from the bullseye at which you were aiming? Would this still be described as a gun shooting a 1 or 2 inch group despite the greater distance of all shots from your point of aim?
(i.e. Your point of aim is irrelevant to your group size.)
(i.e. Your point of aim is irrelevant to your group size.)
1911Tuner
Moderator Emeritus
re:
Howdy RM,
Yep...The group size is what it is, and represents the rifle's accuracy potential. The distance from the point of aim is a matter of adjusting your sighting system to coincide with the point of impact...or better known as obtaining a zero.
If you have a scope with 1/4 minute adjustment, each click or mark
moves the bullet impact 1/4 inch at 100 yards...1/2 inch at 200...
3/4 inch at 300, and a full inch at 400 yards. At least this is the theory.
Some cheaper scopes aren't that honest, and will vary as to how far the impact moves per click. 4 clicks equals one minute...or one inch
at 100 yards...Two inches at 200, and so forth.
A good way to get a general-purpose zero is to adjust the rifle to hit
center at 25 yards, and then shoot it at varying distances out to 300 yards to see where it hits...and make fine adjustments from there, keeping in
mind the "Minutes per Hundred Yard Rule."
In contrast, obtaining a GP zero at 25 yards will require 4 clicks
to move the point of impact 1/4 inch...and 16 clicks for an inch.
My usual practice is to turn the knob one click farther than the amount that I want to move the point of impact, and then back it up one click. This
helps to compensate for any backlash in the turret, which is pretty common
with many inexpensive scopes, and less so with more expensive ones.
There is always SOME amount of backlash, no matter how well-made the scope...The high-end ones just keep it at such a small amount that you'll
likely never see it.
Luck!
Tuner
Howdy RM,
Yep...The group size is what it is, and represents the rifle's accuracy potential. The distance from the point of aim is a matter of adjusting your sighting system to coincide with the point of impact...or better known as obtaining a zero.
If you have a scope with 1/4 minute adjustment, each click or mark
moves the bullet impact 1/4 inch at 100 yards...1/2 inch at 200...
3/4 inch at 300, and a full inch at 400 yards. At least this is the theory.
Some cheaper scopes aren't that honest, and will vary as to how far the impact moves per click. 4 clicks equals one minute...or one inch
at 100 yards...Two inches at 200, and so forth.
A good way to get a general-purpose zero is to adjust the rifle to hit
center at 25 yards, and then shoot it at varying distances out to 300 yards to see where it hits...and make fine adjustments from there, keeping in
mind the "Minutes per Hundred Yard Rule."
In contrast, obtaining a GP zero at 25 yards will require 4 clicks
to move the point of impact 1/4 inch...and 16 clicks for an inch.
My usual practice is to turn the knob one click farther than the amount that I want to move the point of impact, and then back it up one click. This
helps to compensate for any backlash in the turret, which is pretty common
with many inexpensive scopes, and less so with more expensive ones.
There is always SOME amount of backlash, no matter how well-made the scope...The high-end ones just keep it at such a small amount that you'll
likely never see it.
Luck!
Tuner
Leatherneck
Member
"Would this still be described as a gun shooting a 1 or 2 inch group despite the greater distance of all shots from your point of aim?"
Yes it would, using standard small arms scoring. The theory being that you would then adjust your aiming device (sights or scope) to correct the offset and zero on the desired point of impact.
Perhaps a better way to describe the accuracy of the weapon as a whole is to use circular error probable or CEP. That would be the the radius of a circle (with the aim point at its center) that contains half--50%--of the hits.
CEP has the advantage of accounting for both scatter and offset.
TC
TFL Survivor
Yes it would, using standard small arms scoring. The theory being that you would then adjust your aiming device (sights or scope) to correct the offset and zero on the desired point of impact.
Perhaps a better way to describe the accuracy of the weapon as a whole is to use circular error probable or CEP. That would be the the radius of a circle (with the aim point at its center) that contains half--50%--of the hits.
CEP has the advantage of accounting for both scatter and offset.
TC
TFL Survivor
Wildalaska
member
All my shots usually fall into one hole, so its easy for me to measure.
WildnattybumpoAlaska
WildnattybumpoAlaska
Bobarino
member
i use a modifed form of 1911Tuner's method. i measure edge to edge of the farthest holes, subtract a bullet diameter, then divide by 2. using this method, i have become and excellent marksman. 
Bobby
Bobby
RM, you are measuring 'dispersion in minutes of angle'. It just happens that one minute of angle [ MOA ] is almost exactly 1" at 100 yards. Scatter is dispersion and offset is the distance and direction from the center of the group to the point of aim....But you can avoid all that if you follow Wildalaska's method !!
1911Tuner
Moderator Emeritus
Minute of Angle
One minute of angle, taken to 3 decimal places is 1.047 inch at a hundred yards...in case anybody wants to get nit-picky.
RM, if you'll think of dispersion as a cone, with all shots that make up the cone the group. Maximum disperson is at the outer edges of the cone.
The trick is to make the center of the cone coincide with the point of aim at a given range...or zero. Then determine the range that you want your maximum point-blank zero to be. The range that doesn't allow the center of the cone of dispersion to fall below, or rise above a predetermined diameter circle at the bullet's hightest point in the flight path...or the point of mid-range trajectory. The circle is usually 8-10 inches, depending on your intended target. Smaller for varmint-sized critters...larger for deer
and black bear...largest for Elk, Moose, and the big browns.
The maximum point-blank range is the longest distance that your intended target can be engaged without holding over or under the center of the vital zone.
Love rifles!
Luck!
Tuner
One minute of angle, taken to 3 decimal places is 1.047 inch at a hundred yards...in case anybody wants to get nit-picky.
RM, if you'll think of dispersion as a cone, with all shots that make up the cone the group. Maximum disperson is at the outer edges of the cone.
The trick is to make the center of the cone coincide with the point of aim at a given range...or zero. Then determine the range that you want your maximum point-blank zero to be. The range that doesn't allow the center of the cone of dispersion to fall below, or rise above a predetermined diameter circle at the bullet's hightest point in the flight path...or the point of mid-range trajectory. The circle is usually 8-10 inches, depending on your intended target. Smaller for varmint-sized critters...larger for deer
and black bear...largest for Elk, Moose, and the big browns.
The maximum point-blank range is the longest distance that your intended target can be engaged without holding over or under the center of the vital zone.
Love rifles!
Luck!
Tuner
sm
member
I agree with Tuner's posts.
I do have an easier way to cheat. I use clay targets..."If it breaks - its broke". Makes no nevermind if a shotgun, rifle or handgun used. Makes no nevermind of distance of target - With the diameter of a clay, ...close enough for gummit work.
I do have an easier way to cheat. I use clay targets..."If it breaks - its broke". Makes no nevermind if a shotgun, rifle or handgun used. Makes no nevermind of distance of target - With the diameter of a clay, ...close enough for gummit work.
Me too.
LarryoneshotgroupAshcraft
sm
member
Icanmakeclayfallofftargetstandwithouthittingclayatlongrangeandlookgood
If it breaks - its broke
Off hand 300 yds with a iron sighted model 70 in '06...boy did I look good to the onlookers...I hit the stand...they didn't know any different...
If it breaks - its broke
Off hand 300 yds with a iron sighted model 70 in '06...boy did I look good to the onlookers...I hit the stand...they didn't know any different...
Standard group measurement is center to center of the widest holes, which is the same as outside edge to outside edge minus bullet diameter.
Yes, you can get a group size smaller than the bullet diameter; it happens all the time in precision target shooting. Since it is impossible to determine how many shots are in the group, a moving backer is used to weed out the cheaters.
When bragging about group size, it is a good idea to give the range, unless you are a gunzine writer, in which case the idea is to make the gun look good, so groups are shot at 10 feet.
My method of achieving a tight group is superior to those mentioned. I just fire one shot and do a target change.
Jim
Yes, you can get a group size smaller than the bullet diameter; it happens all the time in precision target shooting. Since it is impossible to determine how many shots are in the group, a moving backer is used to weed out the cheaters.
When bragging about group size, it is a good idea to give the range, unless you are a gunzine writer, in which case the idea is to make the gun look good, so groups are shot at 10 feet.
My method of achieving a tight group is superior to those mentioned. I just fire one shot and do a target change.
Jim
Penforhire
Member
Okay, related question, how many shots is a minimum statistically valid group? In pistol and airpistol shooting I tend to go to 10 shots but I've seen 3 and 5 shot groups being reported.
Standing Wolf
Member in memoriam
Thanks, Tuner!
I knew some of that, but can't truthfully say I ever actually understood it.
I knew some of that, but can't truthfully say I ever actually understood it.
Pistol:
Measure center to center on the two rounds farthest apart.
Measuring outside to outside and subtracting caliber will shrink your groups unless you're shooting wadcutters or semi-wadcutters. Other bullet styles will inevitably result in holes in the target which are smaller than caliber--sometimes MUCH smaller.
Even though you have to eyeball the center, it still works out to a more representative result.
Rifle:
Use the outside to outside and subtract diameter. The added velocity of a rifle will generally make a hole that is pretty close to bullet diameter.
Measure center to center on the two rounds farthest apart.
Measuring outside to outside and subtracting caliber will shrink your groups unless you're shooting wadcutters or semi-wadcutters. Other bullet styles will inevitably result in holes in the target which are smaller than caliber--sometimes MUCH smaller.
Even though you have to eyeball the center, it still works out to a more representative result.
Rifle:
Use the outside to outside and subtract diameter. The added velocity of a rifle will generally make a hole that is pretty close to bullet diameter.
Wildalaska
member
Nice..reveal our secret..now everybody will be shooting as good as us
WildnexttimebequietaboutitAlaska
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