Measuring group sizes

[Neutron]

How do most of you measure group sizes? Measure from center of POA to center of each shot in the group and add them up and divide by number of shots in the group? Or center of holes that are furthest apart?

 

[F-Stop]

I used to just do center to center, but it’s much easier and more info now with Ballistic-X. I know some don’t have phones and you do need a known 1” reference somewhere on the target.


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[E.Shell]

Measure the diameter of one bullet hole (outside edges of the "grease mark". This will vary with paper type and bullet design.

Measure across widest part the group (outside edges of the grease mark), then subtract your individual bullet hole diameter. This will be your "center-to-center" group size, and is the standard way of expressing level of precision.

If you're shooting a .308 and use ".308" to subtract for your outside-to-outside measurement, you're exaggerating the group size and because a .308 bullet doesn't make a .308 hole, your group is larger than you think.

Group size (precision) has nothing to do with how close you are to your aiming point (accuracy).

 

[[Kev308]]

Group of 3 or 5 shots, center to center of the farthest hits. If you go by point of aim you are basically measuring how far off your scope is and not the capability of the ammo or your accuracy. If you sight in bullseye and get a tight group, but then switch ammo type and get a tight group two inches higher....

 

[RRomig]

Group of 3 or 5 shots, center to center of the farthest hits. If you go by point of aim you are basically measuring how far off your scope is and not the capability of the ammo or your accuracy. If you sight in bullseye and get a tight group, but then switch ammo type and get a tight group two inches higher....

When using a target like this you can eye ball the group easily because it’s broke into 1” squares. Case in point that’s about a 2 1/8” group so no need to get out any measuring devices unless your buddy is running closer to 2 1/4”. Once I’m on paper l won’t adjust anything until I get a consistent group of 1” max but hopefully and usually 1/2”.
Now that I looked at his caliber I see I was a little off but again that’s not a group I’d bother measuring.

 

[Neutron]

Measure the diameter of one bullet hole (outside edges of the "grease mark". This will vary with paper type and bullet design.

Measure across widest part the group (outside edges of the grease mark), then subtract your individual bullet hole diameter. This will be your "center-to-center" group size, and is the standard way of expressing level of precision.

If you're shooting a .308 and use ".308" to subtract for your outside-to-outside measurement, you're exaggerating the group size and because a .308 bullet doesn't make a .308 hole, your group is larger than you think.

Group size (precision) has nothing to do with how close you are to your aiming point (accuracy).

So you're measuring between the two worst shots? Out of a five shot group it seems one shot is usually apart from the rest of the group. Four shots almost touching and one shot > inch away. That worst shot can occur randomly in the sequence of shots.

 

[[Kev308]]

When using a target like this you can eye ball the group easily because it’s broke into 1” squares. Case I point that’s about a 2” group so no need to get out any measuring devices unless your buddy is running closer to 2 1/4”. Once I’m on paper l won’t adjust anything until I get a consistent group of 1” max but hopefully and usually 1/2.

Haha good point. Just wanted to give an example of my comment.

 

[RRomig]

Yeah a flyer can get in your head. I’ll go from a 3 to a 5 shot group if I get one but it’ll drive me nuts if I’m not sure why it happened.

 

[Uncle Duke]

So you're measuring between the two worst shots? Out of a five shot group it seems one shot is usually apart from the rest of the group. Four shots almost touching and one shot > inch away. That worst shot can occur randomly in the sequence of shots.

It can, and it oftentimes does. Especially later in a shot string, and/or when shooter is aware that the group already on paper is tight, and inadvertantly introduces a variable in technique or breathing or position or cheek weld or other that wasn't there before.

 

[E.Shell]

So you're measuring between the two worst shots?

Yes, "center-to -center between the widest shots" is the correct way to measure a group

Out of a five shot group it seems one shot is usually apart from the rest of the group. Four shots almost touching and one shot > inch away. That worst shot can occur randomly in the sequence of shots.

LOL So is it a "5 shot group" or not?

Tossing the widest shot does have a little merit when testing loads and IF you know that your rifle handling caused the flier. Otherwise, we're just making stuff up.

If you're shooting competition, you can't ignore a lost shot.

If you're shooting for personal enrichment, then figure out WHY you lost a shot so you can work it out.

Uncle Duke makes a good point, and the cause of any lost shots must be determined and corrected.

 

[AlBeight]

I used to just do center to center, but it’s much easier and more info now with Ballistic-X. I know some don’t have phones and you do need a known 1” reference somewhere on the target.


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My Ballistic-X app crashes every time I open it, I’ve never successfully used it. I deleted it just yesterday. My calipers are far better anyway, I just thought the app would be cool, not so much.

 

[Speed3]

Of course Eds advice is spot on.

5 shot group is usually a better indicator of repeatability. You have to determine the cause of the flier and own it. If you discard those without knowing, its a disservice to you.

I've been known to sling 20+ acrosssed a chrono to get better ES/SD data.

Don't get caught up on the tightest groups, lots of ther things factors to shooting unless you're shooting benchrest.