Calculating radius

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cdoc42

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https://www.msn.com/en-us/news/tech...P&cvid=4e538745cb4845b7b2262833ca784c2f&ei=11

I'm not sure how to appropriately include the link to calculating radius vs just measuring group size, but this appeared on my computer from Outdoor Life.

If I misunderstand it, I'm open to correction. But I get the impression one needs to measure center-to-point-of impact of, say, 5 shots, then divide the totals by 5 to get an average.

My major question is, is the point of impact where you expected the bullet to be? If you aimed at the bull from 100 yards and your shots are 3 inches from the bull, meaning you should adjust your scope, or can you measure all 5 from where the first shot landed, regardless of the bull?
 
The way I do it.

My target has 1" grids. For each shot I record the coordinates of the poi. The groups center is the mean of x-, y-coordinates. Calculate each poi's distance from the group center. The median x2 is the R50 group size in inches. I also calculate R90 in similar fashion.

That's for group size, regardless of POA. I also calculate poi's distances from poa (bullseye). From that I can calculate its relevant statistics.

-TL

Sent from my SM-N960U using Tapatalk
 
I have used On Target Precision Calculator software for at least 12 years.
I scan the target after I have shot a session (or you can take a photo).
The software cost me about $12. They have multiple versions that are more expensive, some used for scoring benchrest matches (one shot on separate bulls).

The software allows me to pick the caliber and then calibrate the software by picking a distance on the image and entering the actual measurement. If you were using a target with one inch squares, you would select from one edge of a square to the other and enter the distance in software.

Then you put a circle on each hole of a group and, when you have done that for your 5-round group, you hit calculate and it gives you the center to center group size, in inches and MOA, as well as the width and height from the largest dimension of the group. The groups are measured center-to-center of the 5-rounds and the measured group size is the distance between the rounds that are furthest apart.

I have kept copies or all the groups I have shot since 2010 at 100 and 200 yards with all of my centerfire rifles (just over 10,000 groups) and all my rimfire rifles at 50 and 100 yards (about 3,000 groups).

As for POI versus POA, when I shoot at targets, I purposely have my POI slightly off the POA since my target reticle is aiming at 1/10 of an inch.
If the POI is on the POA, the first shot destroys my POA.
When I am trying to shoot groups under 1/4 inch at 100 yards, I have to have a very precise aim point.
 
I do essentially what tangolima described. My target is a coordinate system with 1" squares (available commercially) with the POA at 0,0. It doesn't really make any difference where the group forms in relation to the POA because that's a question of accuracy which can be taken care of by adjusting the sights - what we're talking about here is precision. There's no pass/fail for number of shots needed, but obviously more shots provide a better estimate of precision. I measure the X,Y coordinates of each shot and then it's a relatively simple matter to calculate the group center (mean X, mean Y) and the distance from that center of each shot using the good old Pythagorean theorem. I have a simple spreadsheet that does all that for me after I input the individual X,Ys. Mean radius, which is a reasonably good estimate of circular error probable (CEP) is then simply the average of all the individual distances.

I've never been a fan of group size (i.e., spread) as a measure of precision. It's easily calculated (measured), but consider that no matter how many shots you have in the group you're only using data from two of them and, as your number of shots increases, the group size can only get larger, never smaller. From a statistical standpoint, both those characteristics are inherently unsatisfying.
 
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