Measuring Group Size

[kanej]

OK -- so this is a very picky question, but its coming from the engineer in me...

When folks reference their group size (usually in MOA) are they measuring the distance between the furthest edges of the farthest apart holes or are the measuring center to center of the furthest apart holes?

As you get down, sub 1 MOA @ 100 yards, it starts to matter. For example, I am working up a load for an AIAE rifle I just purchased used here on the hide. After my first range outing, my best grouping load is 0.85" measured edge to edge, or 0.54" measured center to center.

So the question is, would most folks report this as .85 MOA or .54 MOA?

BTW: A mathematician might re-state the problem this way: what is the smallest group size possible with a a 30 caliber (0.308" diameter) bullet at 100 yards? 0.308 MOA or 0 MOA?

Apologies in advance if this question is asked and answered.

 

[former naval person]

Re: Measuring Group Size

Fire a group at a distance, and fire one other shot on the same paper at the same sitting. Measure the extreme outside of the group, and the single hole, subtract the diameter of the single hole from the group and you have an accurate group size measurement. The diameter of the single hole may and often does differ from the diameter of the bullet. JMHO

 

[waste_knot]

Re: Measuring Group Size

What he said.
Groups are measured center to center. To get your measurement, record the outside distance between the two outermost holes and subtract the diameter of the bullet.

 

[recarga]

Re: Measuring Group Size

(.308") before you measure the groupings , take your caliper open it to .308" or just use a BULLET ones you Got .308" or any given BULLET from where you want to measure . Now zero out your caliper and measure from edge to edge and you will already have your true measure.

Talking MOA
1 MOA = 1.047" at 100 yards but 1.000" is close enough for us.

So if you have a .250" group we say our rifle shots 1/4 of MOA
.500"= 1/2 MOA
.750"= 1/3 MOA
1.000"= 1 MOA

Now this is all at 100 yards but if you pass the 100 yards the MOA (minute of angle ) the measure of the angle will be different as the angle moves away from its center.

So easy enough do this.

Your group size (divided by how far the target was ) times 100= your group size in MOA.

So let's say your rifle shots 1/2 MOA at 100 yards then your rifle will shot 1 MOA at 200 yards ,1 1/2 at 300 yards etc.

You can also apply this to your scope adjusting ,just reverse the calculation .
MOA (times) distance ( divided by 100) = drop of BULLET.

 

[HarryBallz2029]

Re: Measuring Group Size

Recarga 1/3 MOA doesnt = .75" thats 3/4 moa, 1/3 MOA= .333", As others have said the way I do it is measure 2 outer most holes subract bullet diameter etc etc...

Also Recarga this statment is false "So let's say your rifle shots 1/2 MOA at 100 yards then your rifle will shot 1 MOA at 200 yards ,1 1/2 at 300 yards etc."

My Savage 10 FCP shoots 1/2 moa from 100 yds to 600 yds. Just because it shoots 1/2 moa @ 100 has nothing to do with other distances my friend.

 

[recarga]

Re: Measuring Group Size

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DirtyHarry2029</div><div class="ubbcode-body">Recarga 1/3 MOA doesnt = .75" thats 3/4 moa, 1/3 MOA= .333", As others have said the way I do it is measure 2 outer most holes subract bullet diameter etc etc...

Also Recarga this statment is false "So let's say your rifle shots 1/2 MOA at 100 yards then your rifle will shot 1 MOA at 200 yards ,1 1/2 at 300 yards etc."

My Savage 10 FCP shoots 1/2 moa from 100 yds to 600 yds. Just because it shoots 1/2 moa @ 100 has nothing to do with other distances my friend.
</div></div>

1/3 TIPPING ERROR sorry

and on
<span style="font-weight: bold">Also Recarga this statment is false "So let's say your rifle shots 1/2 MOA at 100 yards then your rifle will shot 1 MOA at 200 yards ,1 1/2 at 300 yards etc."

My Savage 10 FCP shoots 1/2 moa from 100 yds to 600 yds. Just because it shoots 1/2 moa @ 100 has nothing to do with other distances my friend.</span>

I should have said this.
on the perffect world this will be true if
there will be no gravity. and the bullet will keep its course.

but i hope our friend got the idea

 

[BuzzBoss915]

Re: Measuring Group Size

Assuming the holes are too close to measure center to center, measure to widest outside to outside and subtract one caliber, .224" or maybe .308". By far the easiest is to measure the outside to the inside of the two holes furthest apart.

There is an interesting variation used many years ago; around the strings; with pegs. I'm sure a few of the old geezers know how its' done. BB

 

[cav.scout]

Re: Measuring Group Size

Furthest edge to furthest edge subtract bullet diameter=group size.

 

[dinc]

Re: Measuring Group Size

As stated above.

Additional thoughts: Group size is measured center to center. 99% of the time just lay some calipers on the most distant centers.

The trouble measuring outside to outside is you have to make sure your outside is a full caliber. If you measure a single .308 hole it will probably be something less and if you use that same method and then subtract caliber your groups will have shrunk.

In competitions get two pieces of clear plastic with a .22 to .3" cirlce drawn on each (both same size). Lay one on the target with it's circle centered on one outer hole. Lay the other piece over the first with it's circle over the other outer hole. Measure. Because you're centering a circle over a circle (the hole) it's easier to align. Measure the left sides of both circles, instant distance. This is the most repeatable method.

MOA vs. inches. Potatoe, patatoe. Just be consistant. Inches are easy.

Guys that say MOA are either so into shooting that they automatically think in MOA or else they are trying to sound cool and/or imply that they consistangly hold MOA at long distances (frequently guilty).

 

[cav.scout]

Re: Measuring Group Size

3,5,10 shots where does it stop. 3 shots is a group anything more is just a challenge to you as a shooter. I shoot 5 shot groups some people think that's not good enough it should be 10, you have to stop somewhere. Only real men shoot 50 round groups!

 

[BuzzBoss915]

Re: Measuring Group Size

Depends. Handgun, I don't mind a 25 round group. Five rounds is the standard, where I come from, for precision rifle. However, in some cases, really hot loadings, I shoot three rounds just to save accuracy for important stuff....like live targets.

But, outside to inside is the easiest because you don't have to do any math.
BB

 

[HarryBallz2029]

Re: Measuring Group Size

Agreed Scoutsout, I generally run 3 shot groups when I am measuring groups from developing loads. Once I have my load I will generally shoot a few 5 shot groups just to say I did and get the measurment for when people ask. 10 shot groups is a waste of ammo imho because by the time you get that far into a group your barrel will be so hot the POI starts changing.

 

[50APIT]

Re: Measuring Group Size

The correct measurement would be from the two furthest centers. There are several different ways you can arrive at the measurement as you have seen. Really, as long as you are consistent with whichever method you choose, you will be able to compare results between sessions.

You can also use software to get some more analysis on group sizes. I have found the On Target software works well and is free.

 

[gunsnjeeps]

Re: Measuring Group Size

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: recarga</div><div class="ubbcode-body">
So let's say your rifle shots 1/2 MOA at 100 yards then your rifle will shot 1 MOA at 200 yards ,1 1/2 at 300 yards etc. </div></div>

No. An inch (1.047") at 100 yards is 1 MOA. 2 inches at 200 is still 1 MOA. So a 3 inch (3.141) group at 300 yards is stil 1 MOA.

Range doesn't change what a minute of angle is.

Also gravity isn't going to open up your groups as you increase range, calling wind and holding aim will. As will the quality of your ammo.

As for the OP, outer edge to outer edge minus caliber works for group size. On Target is good.

BTW: A mathematician might re-state the problem this way: what is the smallest group size possible with a a 30 caliber (0.308" diameter) bullet at 100 yards? 0.308 MOA or 0 MOA?

Answer is 0 MOA. Measurement is center to center no matter what caliber you are shooting.

 

[B3dlam]

Re: Measuring Group Size

Center to Center<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kanej</div><div class="ubbcode-body">OK -- so this is a very picky question, but its coming from the engineer in me...

When folks reference their group size (usually in MOA) are they measuring the distance between the furthest edges of the farthest apart holes or are the measuring center to center of the furthest apart holes?

As you get down, sub 1 MOA @ 100 yards, it starts to matter. For example, I am working up a load for an AIAE rifle I just purchased used here on the hide. After my first range outing, my best grouping load is 0.85" measured edge to edge, or 0.54" measured center to center.

So the question is, would most folks report this as .85 MOA or .54 MOA?

BTW: A mathematician might re-state the problem this way: what is the smallest group size possible with a a 30 caliber (0.308" diameter) bullet at 100 yards? 0.308 MOA or 0 MOA?

Apologies in advance if this question is asked and answered.</div></div>

You want to measure Center to Center as has been mentioned by other people. otherwise a a .308 could never be a 1/4 minute gun which we all know it is capable of doing.

And your engineer mind should know that .54 inches at 100 yards is NOT equal to .54MOA its close but not the same and keep out of that train of thought as it can make a huge difference at ranges past 100 yards.

 

[BuzzBoss915]

Re: Measuring Group Size

Instead of estimating the centers of two bullet holes that are the furthest away, or measuring outside to outside and subtracting the size of the bullet...

I have already given the easiest, simplest way to measure group size, whether you want to convert it to MOA or inches.

<span style="color: #FF0000">First, you determine the two holes furthest apart. THEN MEASURE THE OUTSIDE EDGE OF THE FIRST TO THE <span style="text-decoration: underline">INSIDE EDGE</span> OF THE OTHER. No need to do any math, just measure outside to inside.</span>

It's very simple and easier to judge the edge than estimate the centers.That's the way it's done, folks. BB

 

[MitchAlsup]

Re: Measuring Group Size

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kanej</div><div class="ubbcode-body"> what is the smallest group size possible with a a 30 caliber (0.308" diameter) bullet at 100 yards? 0.308 MOA or 0 MOA?</div></div>

There are benchrest guns under perfect conditions* that will shoot group sizes smaller than 0.1" at 100 yards. This is colloquially known as "in the zeros".

Google "Houston Warehouse" for a good read on perfect conditions.

 

[BuzzBoss915]

Re: Measuring Group Size

....but rarely in aggregate.

Hell, I've shot groups in the "ones", but just lucky, more often than not. On the other hand, I don't own a dedicated bench rig. Maybe I should have a bragging group laminated for my wallet?

I've heard of that Houston Warehouse, but don't remember much about it? BB

 

[John_M]

Re: Measuring Group Size

For years I have been measuring group size the same way suggested by BuzzBoss. For ME, this way is easiest and just as accurate (if not more so)than center to center, outside to inside or outside to outside minus the caliber or inside to inside plus the caliber.

 

[gstaylorg]

Re: Measuring Group Size

Seriously, measure it however you want. Outside to outside and subtract, outside edge to inside edge, center to center (this is what I use), it doesn't matter. Just do it consistently every time. You don't want to be one of those people that quibbles over a 0.01 MOA difference in group size when the rifle (and more importantly, the shooter) simply aren't capable of reproducibly generating group size errors that small.

 

[Fred_C_Dobbs]

Re: Measuring Group Size

A bullet hole is referred to as a "point of impact" for a reason. The operative word is "point" and a point is defined as a position with no dimensions. So the POI is the location of that infinitely small spot on the target where the meplat first made contact. Unfortunately, there is no perfect method for determining the POI because the bullet took the most important bit with it, leaving us nothing but a hole. And there is no guarantee that the POI was even in the exact middle of that hole. So if you're in a competition, measure according to their rules. Otherwise, pick a method that is logical and repeatable and apply it consistently.

But regardless of how you measure them, all that measuring to the two furthest bullet holes tells you is how far away your two furthest bullet holes are, not how large your group is, although it can work out that way.

Here's an example. Start with two bullet holes (represented by red Xs), let's call them POI #1 and POI #2, separated by exactly two inches:

Each grid square is 1"x1". Those two points comprise a "group" that can be described by a circle 2" in diameter:

Now add a third POI, which happens to be almost exactly the same distance from both of the two earlier POIs, and is nearly but not quite 2" from either:

According to the extreme POI measurement theory, this still is a 2" group because the third POI did not increase the group's extreme measurement. But did it increase the size of the group? Let's restore the original 2" circle and see:

Well waddaya know about that? The extreme dimension is unchanged but the original 2" circle no longer describes this group. It obviously now is larger than 2".

I took it easy on myself and just used three POIs because it's easy to determine the "circumcenter" of a triangle. Once you have the circumcenter, you can construct a circle that intersects the triangle at all three vertices. The diameter of <span style="text-decoration: underline">that</span> circle <span style="font-weight: bold">is</span> the true size of the group:

All those extra lines let me determine the circumcenter of the group, then construct a circle that includes all three of the POIs. And the radius of this new circle is 1.15", which means calling this a 2" group underestimated its true size by about 15%.

The extreme dimension method <span style="font-style: italic">can</span> work, provided none of the other POIs are too far from a line that runs through those two furthest POIs, but you've got to examine it in more detail than just eyeballing the target and deciding which two POIs are furthest apart. Otherwise, you're just guessing.

 

[BuzzBoss915]

Re: Measuring Group Size

Fred, Amigo. We don't need no stinkin' badges!

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">
All those extra lines let me determine the circumcenter of the group, then construct a circle that includes all three of the POIs. And the radius of this new circle is 1.15", which means calling this a 2" group underestimated its true size by about 15%. </div></div>

I hate to break it to you, but. Nobody cares about charts and graphs, suitable for a moon shot; they want a simple meaningful measurement. But thanks, for effort, plus a bonus for neatness, you get an A+.
BB

 

[John_M]

Re: Measuring Group Size

In Official Competitions do "group size" and "circle size" have the same meaning? Which process is used to determine the winner of the competition-variations on the center to center measurement most of us use or the triangulation method presented so professionally by Fred?

 

[cyberpuppy42]

Re: Measuring Group Size

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 50APIT</div><div class="ubbcode-body">You can also use software to get some more analysis on group sizes. I have found the On Target software works well and is free.</div></div>

This, for me. Takes all the guesswork and math out of it, and it's a standard measurement for MOA as far as I can tell. You need a scanner, but most people have one built into their printer or their cell phone (i.e. take a photo of your target). Very simple to use.

 

[BuzzBoss915]

Re: Measuring Group Size

Those computer wennies find a way to screw up everything. <sigh>