Maybe this is just news to me, but VarmintGuy gave me a good piece of advice for calculating group size that I think is worth repeating...

After measuring from outside the black ring between your two farthest shots, measure a single bullet hole on the target to get the actual diameter as the bullet pierced the paper. At least in my case, rather than being .224", which is what I expected, it was exactly what he suggested it was -- .210". Strange, but true. Even though this made my group size larger and decreased my bragging rights by a bit, it is a more accurate measurement. Besides, .213" still ain't that bad...

Anyway, VarmintGuy...thanks again for the pointer.

Got_the_bug,

Don't want to rain on your parade, but you have been given duff advice. Just how does a 0.224 bullet lose 14 thou of diameter between when you loaded it in the cartridge and when it hit the target? Measure the paper you are only measuring the disturbance the bullet made as it passed thru. The standard group size is the distance between the two furthest edges of the holes of the group, less 1 bullet diameter.

Pete

Pete Millan: How about this perspective - we can all figure out a group size when the furthest bullet made holes in a group are seperate and complete outlined circles. So measuring from edge to edge on whatever diagonal the furthest two holes are using same edge to same edge for this simple group measurement. Bullet hole size does not matter here to find center to center group size here as edge to edge is the same as center to center!

But when all shots are within one hole it is then only possible to measure outside edge to opposite outside edge of whatever holes are partially outlined. Thus to find the center of those holes we need to know what size hole a bullet makes in the kind of paper we are using! Then what ever that measurement is you subtract the measured "bullet hole size" from the furthest edge to opposite furthest edge group size to then obtain the center to center group measurement.

I hope I explained that better this time.

Maybe this will aid you - visualize me or got the bug shooting a two shot group! Now these two shots make a real small looking one hole group! And, as we said, the furthest edge to opposite furthest edge measurement in mine or got-the-bugs paper target measures .215"! So we go to a single bullet hole somewhere in this same kind of paper and we see that it measures .210"! If the single hole is .210" then we know the .215" hole must be a group! And if we were to follow your advice (subtracting .224"!) we would then have an amazing 2 shot group of (hold on to your hats!) -.009" (minus .009"!)! And that can't be done - right! So yes you need to know exactly what size hole your bullets are making in your paper to find the exact center to center group size for "one hole" type groups!

Got the bug or my correct two shot group size is of course +.005" for the above illustration!

Please let me know if you disagree or do not understand my explanation - I will try harder next time.

Hold into the wind

VarmintGuy

Pete...

I know it doesn't seem to make sense...how does a .224" bullet make a .210" hole? I think that you said it well when you said it was the "disturbance" that the bullet made in the paper. Aren't we actually measuring the "disturbance" in the target, though? In my case, if I subtract the diameter of the bullet (.224") from the two farthest holes (.423"), I get a group size of .199". If I substitute the actual size of a single bullet hole on the target (.210") for the static bullet diameter, I get the larger group size of .213"...is this not a more accurate way of target measurement? It seems to me that to subtract a bullet diameter of .224", you are assuming that your bullets are making .224" holes in the target (which, in this case, mine are not).

I can see what you two guys are saying, but any benchrest outfit that I have ever heard of uses the actual bullet diameter, and always have. I shoot score, not group and could be wrong, but any group measuring devise that I have seen measures the size directly and does not need any math to figure it out, It does however figure out to be extream spread minus bullet diameter. Bob

VG, if you measure a 0.215" two hole group edge to edge and you are shooting a 0.224 cal rifle, then a legion of BR shooters would probably tell you that:

a) You have not measured correctly, or
b) You are cheating, or
c) You are the new messiah

I understand the math of what you are proposing, in that you are measuring "relative" to the actual bullet calibre, and you are introducing the same "error" in measuring the group as you are in measuring the hole, in order to get the center to center measurement.

Point is there is one standard method of measuring groups, we don't need a second one!

Cheers

pete

Not saying it is right or wrong but the smaller hole is from the paper stretching or rebounding (for lack of a better description) back a small amount after the bullet has passed through like the tears around the black ring.

Pete Millan: Well you are correct on one small point! The correct way to find the center to center group size is to first determine where the center is! If you are making the incorrect assumption that a .224 caliber bullet will only make a .224" size hole in a good quality paper or official Bench Rest target then that is the root of your misunderstanding.

Those of us who most often use linear measuring instruments to measure group size must ascertain where the centers of the furthest shots are. And to find those centers correctly we must know the size hole ones bullet makes in ones target! If a person assumes the partial outline of a bullet hole is a .224 hole when in actuality (like I and Got the bug have noted) the bullet is only making a .208" or .210" hole then you do that math for yourself and compute your "built in error"!

I have seen hundreds and hundreds of sanctioned Bench Rest Match targets measured. The measurer uses a sliding device that super-imposes two circles and the measurer adjusts the device till the 6mm or 224 caliber holes find the largest dispersion of the group. This device measures in thousandths. The extreme diagonal direction is marked in pencil outside the group to show the direction the measurer feels the greatest dispersion has occurred and the result in that direction is measured and called group size.

I suggest you go shoot a .224 bullet through a target and measure it! Then think about what you have obtained and reconsider your position.

It is so easy to do and granted it only makes several thousandths difference in doing it correctly (my way) or in doing it incorrectly (your way). But because it is so easy to do correctly I suggest you learn to do it!

But again, first to convince your self please shoot a good quality target with various caliber bullets and measure them! Then compare your measurements with the various bullet diameters. It will be gratifying for you I am sure.

I claim not to be a Messiah! Just someone who learned this lesson a long, long time ago. I shoot a lot!

I simply can not understand why it is so hard for you to grasp this finding centers of the bullet holes concept! I mean the center of a bullet hole is the center of a bullet hole! And our object in measuring group size is to find the center to center measurement! The center is the center is the center. Your mental error is when you are using the wrong size (in thousandths of an inch) to subtract from the extreme edge to edge measurement of a one hole group! In mine and Got the bugs (everyones?) targets your method of group determination gives an inaccurate measurement of up to .013" or .014" and a minimum inaccurate measurement of .007"!

I hope this latest post will shed some light for you! Please go back and re-read my last posting again. As I have had a couple of offline E-mails thanking me for explaining it thusly and now those folks are measuring correctly by not assuming a .224 bullet will make a .224" hole in a target!

Cheers back at ya!

Hold into the wind

VarmintGuy

So with all that said: Though the size of the hole a bullet makes when passing through paper never reduces, the distance between the points of impact on the paper by subsequent bullets (group size) can certainly reduce, since it is a relative measurement, right?

BTW, center to center is how I measure my groups. Actually, when using steel plates, I can measure from point of impact to point of impact based on the very small divit the bullet makes in the steel.