Measuring Groups

jaycee

Just wondering if I can get a steer in the right direction on this ...

When measuring groups I have seen some people refer to
the center to center size (Where the bullet diameter is subtracted from the total group size)

while some use the edge to edge yardstick.

Which is correct ..?
Or does it vary ?

BTW..
I appreciate a tight group as much as the next ,
but it's becoming the standard by which everything is judged ...
I've seen it illustrated in countless gun reviews, the writer provides a picture
of a close knit group of holes..lovely..but off target .

I (Being an old fashioned type) tend to think that the whole essence of
markmanship is hitting what you are aiming at ..i.e.. the center or bull .

Sparks

Edge-to-edge is all I've ever seen used in ISSF target shooting, but center-to-center and edge-to-edge should be pretty much the same thing, differing only by the diameter of the bullethole. That might be a bit hard to measure very accurately given the way different bullets tear the paper, of course.

And the group is everything. If you can get the shots to all land in the same place when you pull the trigger, but it happens to be a centimetre to the left, well, you just adjust the sights a few clicks to the right and now all the shots go through the centre. Whereas if you can't get the shots to land in the same place every time, how do you know if you should adjust left, right, up or down? And if it's not you, but the gun, well - why would you buy it if you want to do target shooting? (I mean, if it's the rifle your great-grandad used in WW1, that'd be one thing, but if you want a rifle to do target shooting or hunting, you want to know the thing won't send bullets in a slightly different direction every time, y'know?)

Of course, if you're scaring bunnies or stalking deer, this may not apply, since you're only meant to take the one shot, maybe the hunters could give their perspective?

jaycee

Hi Sparks ,

I think you misunderstood my point..

Edge to edge @ 50 yds a 12 mm group represents roughly 1 MOA
Subtract 6mm (.22Lr) and it becomes a 6 mm group ..Or .5 MOA ..

Big difference.

This isn't a gun I read about in a gun review with a view to buying ..
that was just a general comment about how such things are presented and actually it serves to illustrate my point .. If a rifle is advertised as accurate to
.5 MOA ... what does that mean ..? what standard of measurement are they using ..? From the example above it could be the difference between accurate ..and just ..Ok for rabbits.

Sparks

Ah, I see what you mean jaycee. Yes, if they're presenting it as MOA with no reference to whether or not it's edge-to-edge or centre-to-centre, that would be a problem.

civdef

It's easy, quote which ever figure makes you seem like a better shot...

That said, it seems more logical to use the centre to centre value. Otherwise you couldnt get anything smaller than a .5" group with a .5" rifle, for example. That's fine if you're just comparing rifles of the same calibre, but would make accuracy comparisons between a .22 and a .5 difficult.

jaycee

It's easy, quote which ever figure makes you seem like a better shot...

Now there's a concept i'm familiar with..!

Interesting idea too..subtracting the bullet diameter of .5 from a .5 group..

Rovi

jaycee wrote:

Interesting idea too..subtracting the bullet diameter of .5 from a .5 group..

Well......................
a. 1 single bullet through a target is a 0.0 MOA 'group.'
b. As is an infinity of bullets through the same hole.

I can do 'a.' any time, at a moment's notice, with any gun.
'b.' may take longer

Until Sparks mentioned the ISSF method of group measurement (edge to edge), I've only ever heard of groups being measured from centers.


1 MOA and better rifles are getting into target/sniper country, and incremental improvements of 0.1 MOA are usually measured in vast sums of money, particularly when you get down around the 0.5 MOA mark.
Hunting rifles are usually considered adequate if they can shoot 1 MOP (Minute Of Prey)
MOP on a rabbit would be 3-4 inches across (chest/head areas) at whatever range you've decided is reasonable.
Its bigger on a fox, and bigger again on deer, obviously (all at the same range!).

So, in theory, a 4 MOA rifle is good enough for rabbits at 100 yards.
Provided, of course, that you can hold EXACTLY in the centre of the target zone! Not easy, particularly in the field where you don't have a convenient benchrest

Pretty much any un-abused decent quality rifle will shoot 1-1.5 MOA, and is thus plenty good enough for hunting at reasonable ranges on suitable prey.
For the vast majority of shooters, the difference between a 1.5 MOA rifle and a 0.5 MOA one will only show up on a target bench, except when it come to carrying the thing around the woods or fields.
The 0.5 MOA rifle will invariably be heavy and awkward compared to a conventional hunting rifle. It will have a weird stock, a very heavy barrel, and a super light trigger that would be positively dangerous in the field.

Of much greater importance to a hunter are good tracking/stalking skills, good range estimation, and an intimate knowledge of first shot placement from a cold barrel.


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gouda

Rovi wrote:

Well......................
a. 1 single bullet through a target is a 0.0 MOA 'group.'
b. As is an infinity of bullets through the same hole.

I can do 'a.' any time, at a moment's notice, with any gun.
'b.' may take longer

Until Sparks mentioned the ISSF method of group measurement (edge to edge), I've only ever heard of groups being measured from centers.


1 MOA and better rifles are getting into target/sniper country, and incremental improvements of 0.1 MOA are usually measured in vast sums of money, particularly when you get down around the 0.5 MOA mark.
Hunting rifles are usually considered adequate if they can shoot 1 MOP (Minute Of Prey)
MOP on a rabbit would be 3-4 inches across (chest/head areas) at whatever range you've decided is reasonable.
Its bigger on a fox, and bigger again on deer, obviously (all at the same range!).

So, in theory, a 4 MOA rifle is good enough for rabbits at 100 yards.
Provided, of course, that you can hold EXACTLY in the centre of the target zone! Not easy, particularly in the field where you don't have a convenient benchrest

Pretty much any un-abused decent quality rifle will shoot 1-1.5 MOA, and is thus plenty good enough for hunting at reasonable ranges on suitable prey.
For the vast majority of shooters, the difference between a 1.5 MOA rifle and a 0.5 MOA one will only show up on a target bench, except when it come to carrying the thing around the woods or fields.
The 0.5 MOA rifle will invariably be heavy and awkward compared to a conventional hunting rifle. It will have a weird stock, a very heavy barrel, and a super light trigger that would be positively dangerous in the field.

Of much greater importance to a hunter are good tracking/stalking skills, good range estimation, and an intimate knowledge of first shot placement from a cold barrel.


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That was a wind up post,wasn't it? One MOA is approx. 1.047" @100yds,2.094" @200yds, 3.141"@300yds etc. as explained below.
Minute of Angle

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Most firearm sighting scopes incorporate windage and elevation adjustments referenced to MOA (minute of angle). Each click of the scope turret is usually 1/4 MOA change and on some scopes 1/8 MOA. Normally, shooters refer to these adjustments as a change of a fraction of an inch at 100 yards rather than the true value of MOA for which they are supposedly calibrated to. The value of inch is a nice easy number to work with and most of us can easily visualize its length and its multiples without the aid of a calculator. Actually the comparison is close enough to not be of practical concern, especially at distances up to a few hundred yards, and the real difference is a mere 0.47 inch at 1000 yards. For serious target shooting and as shooting distances increase the attention to MOA value relative to sight adjustment becomes more essential.



Calculating Minute of Angle
The angle of an arc is expressed in number of degrees. There are 360 degrees of arc to a full circle. Each degree consists of 60 minutes of arc. The distance covered by the measure of arc is relative to the circumference (total distance around the circle) it is contained within. Knowing the radius (distance to center of circle) circumference is easily calculated by using the constant pi . The ratio (represented by pi ) of circumference is constant to diameter (radius x 2) regardless of circle size. The precise value of pi is so far unknown to man but is normally resolved to 3.1416 or 3.141 for our purposes.


Suppose a circle with a 6 inch radius. Circumference can be calculated as:

circumference = (radius x 2) x pi
circumference = (6 x 2) x 3.1416
circumference = 12 x 3.1416
circumference = 37.6992 inches

The distance covered by 1 degree of angle (37.6992 / 360 or, circumference divided by 360 degrees) is 0.1047 inch at 6 inches from center of circle.
And, 1 minute of angle represents (0.1047 / 60 or, 1 degree divided by 60 minutes) 0.001745 inch at 6 inches from center of circle.

Knowing what MOA represents allows us to calculate its value to any distance.
Six inches (the radius of the above example) is 1/600th of 100 yards: (100 yards x 36 inches) / 6 inches = 600
Therefore, the value of MOA at 100 yards is 1.047 inches (0.001745 x 600 = 1.047)
At 50 yards 1/2 the 100 yard value; 70% @ 70 yards; twice @ 200 yards; 6 times @ 600 yards; and so on.
So, the difference between thinking in inches as opposed to MOA is 0.47 inch @ 1000 yards.



Four (4) clicks of the scope adjustment equals 1.047 inch change @ 100 yards for scopes of 1/4MOA per click.


Where group size is expressed in inches the word 'inch' should be spelled. Writing the symbol commonly used to represent the measure of inch (") is not accurate here, and in fact misleading since that symbol also represents 'second of angle' (1/60 of a minute of angle). Therefore, a group of 1 inch would properly be written as '1 inch' not 1". It could also be written as 1' since that symbol (') is used to represent minute of angle but that might be misleading to those thinking in feet.

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Why Use MOA
As shown above, the value of a measure of angle can easily be calculated to any distance from its source of origin - center of circle or muzzle of barrel. Such measures as MOA (minute of angle) are also part of our only universal language - mathematics. While knowing a rifle has a precision of a certain value of the inch at a particular distance also makes it simple to calculate its precision at other distances, one would have to know both the distance and the measure in inch (example: 1 inch @ 100 yards) before calculating it to other distances. In contrast, knowing only the measure of angle is needed to do the same. An example of the contrast is expressing 0.73 inch @ 100 yards verses 0.7 MOA (0.73 / 1.0472 = 0.697). The two equate practically the same (1 MOA = 1.047 inch @ 100 yards) but the expression using MOA is more concise since no distances are included in the expression.

Two informative sites for further discussion of MOA relative to firearm use: What Is MOA and Is It Really an Inch At 100 Yards? and Mil-dots and Minutes-of-angle, From a Technical Perspective

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Calculating Scope Click
For long ranges where shooting distances may vary considerably it is wise to know the actual value of each scope adjustment. This is especially true if scope settings are changed in the field as shooting distances change. Not all scopes are precisely calibrated to MOA (minute of angle) or to the inch. Below is a procedure learned from Varmint Al's Shooting Page for better determining the value of each click of the scope turret.

Shooting from a solid bench rest, determine the center of group using the Average Group Radius method. Without reaching the adjustment limit of the scope, make and record as many scope elevation clicks as will still keep the group on the target while shooting at the same aiming point as before. Again determine center of group using the Average Group Radius method. Return the scope elevation to its previous setting. Determine the distance between the center of the two groups. Divide that distance by the number of elevation clicks used to achieve group two. That is the calculated value of change for that distance for each click.

For high power rifles it is recommended the target be no less than 100 yards distance as some projectiles may not completely stabilize at shorter distances, thereby giving a false indication of true performance.

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Rovi

Gouda wrote:

That was a wind up post,wasn't it?

What makes you think that???


Seriously, I thought I made reasonable points there.
Have I managed to get both feet in my mouth simultaneously, and somehow not noticed?


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