How do you measure group size?

[dmftoy1]

Is it from the furthest outside edges of the two shots that are the furthest apart, or is it from the center of the two shots that are the furthest apart? I'm just curious because I was watching a "Sighting-in wiht Shooting USA" show and he was measure a group with calipers and seemed to be using the centers ... I've always measured from the outsides.

Regards,
Dave

 

[Jim Watson]

Center to center of the two widest shots.

Since it can be difficult to find the center of a hole, you can measure outside to outside and subtract one caliber. Or, if it is not a one ragged hole group, measure from the inside of one wide shot to the outside of the other.

 

[cmb3366]

You measure the extreme outside to outside distance, then subtract 1 bullet diameter from the total. This gives you the exact center to center distance, and removes the "optimisim" that ususally gets interjected when people try to 'eye' the centers of the holes.

 

[taliv]

right. if you measured outside edge to outside edge, then even if you placed each bullet exactly on top of the others, a 50bmg would never be able to do better than 1/2 MOA at 100 yrds.

 

[Kharn]

Center to center, then subtract 75% for when I talk about it on the internet.

Kharn

 

[Bartholomew Roberts]

The key to a good internet group is to just mention the group size without mentioning the match ammo, match barrel, bench, 24x scope and 15 sandbags you used for support. It sounds so much more impressive that way and no exaggeration necessary!

 

[dmftoy1]

LOL . . thanks!!

 

[longhorngunman]

What's considered the standard number of shots for a group? I've shot right at 1 MOA at 100yards with a K31 but this was only using 3 shots. Never been that accurate with 4 or 5. I thought I read somewhere that a "real" group for determining a guns accuracy is 5 shots.

 

[rockstar.esq]

There is no consensus however I have noted on many occasions that it hovers around five shots as this is the most common capacity of a rifle.

 

[Bartholomew Roberts]

If I am trying to shoot groups to get an idea of the potential of the rifle, I always shoot at least a five-round group. It is difficult in that you have to be disciplined for a longer period of time; but if you can put 5 shots in the same place, it greatly reduces the chances that it was a fluke or luck.

I'll use three round groups for sighting in; but beyond that I think you need a minimum of five rounds to get a realistic idea of what the rifle and shooter are capable of.

 

[taliv]

it depends entirely on your motive

for sighting in, 3-shots
for internet bragging, 5-shots
for really finding out how you and your gun shoot, 10-20 shots

i'd rather have a tight 20-rnd group with maybe one or two fliers than a one-hole 5-shot group

 

[Dr.Rob]

Outside edges of the holes. It's not a half inch group unless you can cover all three holes with a dime.

Now there is a difference with how your RIFLE can shoot and how you can shoot. IE sandbagged, benched, bolted down.

I usually use 3 shot groups, and a number of them to 'sight in.'

5 shot groups can heat your barrel up too much and create 'stringing.'

Fire 3 shots, let it cool off, fire three more, cool off, fire 3 more. Again, if the point is to see how well the RIFLE shoots. This of course depends on the profile of the barrel and caliber.

 

[cavman]

As I am a beginner my group sizes usually begin with the question:

"Is it bigger than a bread box?"

Other than that, I refer to rings: all better than the 10 ring. All in the 9-ring, ect. at 10 yards or less.

Pie-plate accurate at 25 yards...saucer plate accurate at 25 yards....

I usually throw down 40-50 rounds and then see how accurate I am.

cavman

 

[cslinger]

Outside edges of the holes. It's not a half inch group unless you can cover all three holes with a dime.

Agreed, this is how I do it and I also tend to use coins to compare groups.

 

[dm1333]

I take one shot, the barrel never overheats that way. My groups run from .22 to .311, .40 with my Glock. I am the anti internet braggart, my groups are measured edge to edge with no subtraction.

 

[Kevlarman]

You could always get one of these handy things:

If you count all 5 shots, it's a 3" group. Count 3, and it's 1/2"

BTW, that was an AR-15 at 100 yds.

 

[JohnKSa]

Since it can be difficult to find the center of a hole, you can measure outside to outside and subtract one caliber.

This is ok for relatively large groups, but when the group size shrinks down close to the caliber size, the relative error of this method becomes large.

I've never seen a rifle bullet hole in paper that was the size of the nominal caliber. The holes are almost always a good bit smaller than what one might expect.

In spite of the fact that "eyeballing" is required to guess at the bullet hole center when measuring center to center, the "eyeballing the center of the hole" method always gives a more accurate number than measuring outside to outside and subtracting caliber.

As an example, the last time I went to the range, I was fortunate enough to shoot a pretty good group at 50 yards with my CZ-452.

Measuring center to center, I consistently get 0.29" BUT, if I measure from outside edge to outside edge I get 0.405" and when I subtract .224" (nominal bullet diameter) and then round to 2 digits, that gives me a group size of 0.18"

That's about HALF the actual size of the group. Where does the error come from? I measured one of the holes shot on identical paper and with the identical ammunition and rifle--and it's NOT 0.224" in diameter. In fact, it measures ONLY 0.12" in diameter. That's about half what one might expect based on the bullet size.

If I measure outside to outside I get 0.405" as before. Then when I subtract 0.12" (the actual hole size) from that number and round, I get 0.29" which is the same as the center to center measurement even though I got that measurement by "eyeballing" the center of the holes.

If you're going to measure outside to outside, you need to subtract the ACTUAL hole size as measured on the paper, NOT the nominal bullet diameter. Otherwise you're almost always underestimating the size of your group--perhaps by as much as half the bullet diameter.

 

[dfaugh]

You measure the extreme outside to outside distance, then subtract 1 bullet diameter from the total. This gives you the exact center to center distance, and removes the "optimisim" that ususally gets interjected when people try to 'eye' the centers of the holes.

that's the way I've always done it.

I shoot 10 shot groups- That proves that both myself and the gun are getting the accuracy. I've shot the occasional 3 and even 5 shot group that was teeny-tiny (which usually means the gun is accurate and/or I'm having a "lucky day")...But if I can shoot 10 shots into the same hole, then I know I'm doing my part as well...

 

[redneck2]

for internet bragging, 5-shots

this obviously an error. You pick the closest two out of ten. The others are flyers

actually, if it's a centerfire (and you handload) you can take two bullets and put them in the two furthest holes and measure to the outside, subtract a caliber.

If it's for internet purposes, subtract two to five calibers unless it's less than zero. This is the method used to calculate the "shoots 3/8" all day long" groups

 

[Davo]

What Jim said. BTW I took my k31 out last week and shot 6 3-shot groups with my new scope setup. The smallest group was .89 MOA, the largest 1.5 MOA, the average of six groups was 1.05 MOA. That is with a standard issue military rifle that is over 50 years old.