Measuring groups?

[nvbroncrider]

Been looking at some groups that others have been posting on the forums and it came to mind that some of them look a little larger than what they seem to measure. I normally just take my cailpers and measure the outside diameter of the group is this right or wrong?

 

[rick smith]

Outside to outside and subtract a bullet's diameter.Rick.

 

[DrMike]

Outside to outside and subtract a bullet's diameter.

This is the standard, taking care to ensure that you are measuring the impacts representing the farthest extreme observed. I measure to the ring on the outside of each hole rather than the edge of the torn paper, just to ensure that the report is as accurate as possible.

 

[nvbroncrider]

I shot a group with my ar-15 that measured .56 on a caliper and then subtract the .224 to get a .336 inch group. Not bad for factory ammo.

 

[DrMike]

Not bad at all. To gain a true picture of what a particular system is capable of giving, I recommend averaging several three-shot groups. Better yet is to average several five-shot groups. The old rule of thumb is that you need a minimum of ten shots to have a statistically significant data source. Nevertheless, because we seldom shoot ten shots in the field, most of us use three shot groups. At some point, however, it is good to do the math so that you can speak of what your rifle is capable of doing. In the field, you have to consider the variants whenever you pull the trigger.

 

[nvbroncrider]

I got to test some handloads for it the other day and then put together 195 so that should keep me busy for a while but i'm gonna try some v-max and bt in it to see what shoots the best.

 

[ROVERT]

Outside to outside minus the bullet diameter is very rarely an accurate way to measure groups. The hole printed is typically well under bore (or groove more precisely). The .277" holes on the following target, for example, average .258" in actual measurement. If you are going to measure outside to outside, you should then subtract the average printed hole size to get your center to center measurement.

I find that either of the two following methods work out more accurately:

Outside to inside:

Center to center:

While the center to center method involves "eyeing it up", I've found that the human eye is capable of getting very close... much closer than subtracting a bullet diameter that is often off by 20 thous.

 

[Elkman]

I agree that more shots in a group give better statistical results, except when you inject the greatest variable, the shooter. As my shot number increases my desire and tension increases to keep the string going. In addition as with most of us my tolerence for 40 pounds of recoil seems to wane after 10 or so shots. I like the average of a number of small groups unless you machine rest the rifle. Then you actually know what the rifle will do.
On modest recoiling rifles the above may not be as significant, however I still find my self tensing up when I start seeing sub dime sized groups beginning to show. I then rush the shot invariably, and yes I shoot a lot but sometimes can't overcome my brain. Just my .02

I measure outside to outside and subtract for diameter.

 

[6mm Remington]

I'm inside to outside. I think all of us panic a bit when we start seeing a perfect group down there and think..........Is this the one that's going to blow it? When I shot that last group with my 30-06 recently and I could not see the hole get any larger after the second shot, no there was no pressure at all to perform my part at least! :mrgreen:

 

[Guest]

If you've got a computer (assuming everyone on this board does...) and either a scanner or digital camera - even a cell phone camera - you can use OnTarget to give you very good measurements of your groups. It's a free package that's a simple download. Read the writeup here:

http://www.6mmbr.com/ontargetsoftware.html

I've been playing with it for the past couple of months and really like it. Once you figure out how to use it, it's easy and fast.

 

[Scott Spencer]

Center to center:

While the center to center method involves "eyeing it up", I've found that the human eye is capable of getting very close... much closer than subtracting a bullet diameter that is often off by 20 thous.

This is the exact method I use. I figure the center of the POI is the best place to measure from.

 

[SJB358]

I don't like to measure! Who really wants to know how bad they are shooting!!! :lol:

 

[nvbroncrider]

It is easier to go from the outside of one hole to inside of the other. That measurement will give you the same as the center to center measurement but is a lot easier to get don't have to worry about human eyes then. That's how you do it when fabricating.

 

[Oldtrader3]

I am not really very anal about it and just measure with an Archetic's scale (in 1/16's), center an inch on a hole (to start) and round up to the nearest next higher 1/16th of an inch on the farthest hole that I measure to. I shoot mostly three shot groups because my rifles are all hunting rifles with sporter weight barrels, I am too old, brain damaged and shaky to care about (5) shot groups. Plus, most of my scopes are hunting scopes, up to 10 power. Groups to me are more meaningful to me if they are consistently similar in size (and small).

Plus, if I wanted to do statistics on groups, I would do a 30 shot ANOVA anyhow. Mainly, I just want to know how my loads are doing group wise versus historically and chronographically..