Group Size and MOA

[Newbie]

I know that shots groups are usually measured in groups of at least five shots. However, I have a simple question about three shot groups I can't find the answer to and I was hoping someone could help me.

Let's say the first shot is perfectly centered on the bullseye. The second shot is perfectly aligned from left-to-right, except is exactly 1 inch lower. The third shot is perfectly aligned from left-to-right except is exactly one inch over the first shot. The line is two inches long, except the max spread from the center of the group (bullseye) is 1 inch. Every shot is no more than 1 inch away from the center. If you made a circle, it would be 2 inches in diameter and 1 inch in radius. Is that 1 MOA or 2 MOA?

Would that be referred to as a 1 inch group or a 2 inch group?

[ 08-24-2004: Message edited by: Newbie_71 ]

 

[Dave King]

Good one!

You just shot a 2 inch group or about 2 MOA. Just to confuse the issue more your precision is about 2 MOA but your accuracy is about 1 MOA.
http://bell.mma.edu/~jbouch/Glossary/Precision.html

 

[preacherman]

...to calculate group size (3 shot, 5 shot, whatever), measure the farthest distance between the outside edges of the bullet holes and subtract the bullet diameter...

 

[Newbie]

Thanks Dave. Now, let me start out by saying that I don't question your answer as to whether or not it is correct. I only have some Newbie questions as to being curious as to the way things are commonly done. If I put 9 shots inside of 1 inch at 100 yards and the 10th shot is 12 inches out there, I have a 12 MOA group? If I go to the range the next time and spray them all over an 11 inch area, I have an 11 MOA group no matter how they are dispersed?

I wish there was a percentage associated with the MOA, such as I am 90% 1 MOA, 70% .5 MOA, 99% 2 MOA etc. Of course we would have to establish whether we were measuring from the center of the group or the center of the bullseye.

 

[Varmint Hunter]

Groups are always calculated for center of bullet hole to center of bullet hole. This way a group fired from a .224 cartridge can be fairly compared to a group fired from a .338 cartridge (or any other). Measuring the outside edges of the farthest bullet holes and subtracting the bullet diameter is just the most common way of determining the center to center measurement.

For informal measuraments you can easily lay a dial caliper across the two farthest bullet hole centers by eye and you won't have to bother with the math. Its fast and easy for comparing your various group sizes.

Using a 5 shot group for measuring is the norm but some guys prefer 3 shots with the magnum cartridges because they get hot quickly and this can lead to rapid throat erosion. Generally speaking, 5 shot groups give you a much better idea of a loads consistency. It is easy to be fooled by an occasional good 3 shot group that may not really be a consistent performer.

Then there are the guys who shoot sub .25" groups with there box stock deer rifles. Some of these guys shoot half a box of ammo but only measure the 2 closest bullet holes. All the other shots are chalked up to "I must have pulled that one".

VH

[ 08-25-2004: Message edited by: Varmint Hunter ]

 

[Dave King]

Mewbie_71

I can see your point and it's about the first time I've ever seen it stated on any forum in such a short and concise manner.

Accuracy and precision are for me a paired set and if one has great precision and little accuracy there could be a giant problem.

I believe that the desire we have for precision is good, it in a way gives us the percentage you mention. I know my rifles will group .5 MOA and with me shooting them they'll still group 1 MOA (precision). As I add variables my accuracy decreases faster than my precision, the most significant variables being wind and mirage. I believe my standard field capable accuracy is 1 MOA (plus or minus 1 MOA from Point Of Aim to Point Of Impact, a 2 MOA group of sorts) at the distances I hunt.

In the real world I believe your scenario of the 11 shots to 1 MOA and the 12th to 12 inches is more an accuracy issue than precision... I can and do call shots bad on occassion... these are accuracy issues ("do overs" or invisible shots while at the range).

The 12 shots to 11 inches (scattergun pattern) is an unknown, either bad precision, bad accuracy or both. At this point a second (competent) shooter would be required so sort the mess out.

Over time many of us have established our own personal percentages. We are often comfortable with them and have confidence that we can continue to maintian the percentage... Confidence is what I call it.


One thing that amazes me while I bantering about precision and accuracy is the issue of having a .5 MOA rifle and then sighting it 2 inches high at 100 yards and leaving it there. Precision is there but accuracy is nearly completely dismissed, the paired set is now broken, a shame of sorts.

 

[Aussie]

Dave ,
I can see where you're coming from but...
For those that shoot out to say maximum 300 yds with a flat shooting calibre , sighting in a couple of inches high at 100 saves a lot of messing around with scope adjustment. Remember that game doesn't always stand around and wait for you to make scope adjustments and that most hunters don't use scope reticles that provide more than a single aiming point . Sighting in a couple of inches high is a reasonable compromise for those not interested in long range hunting .
All but the smallest varmints will succumb to a shot that falls within 2 inches of the desired aiming point .

 

[Dave King]

Aussie

I agree that there is an advantage to having a "walking around" setting. I set my big game rifles to 2 MOA up and my varmint rigs to 1 MOA up when I'm just out floundering about but when I need accuracy and the critter decides to stay put I adjust elevation to the correct distance (usually in the 250 to further range unless I'm getting really picky or for a brain shot).

I guess that last remark in my previous post was more for the fellas that shoot their whatever to 400, 500, 600 and further by guessing holdoff(s) or without any knowledge of that's happening. (A little tiny group two (2) feet below the critter isn't much good.)

 

[Aussie]

Dave ,
No arguement from me .
If Leica rangefinders weren't $1000 here I'd have one and do the same as you do . Fortunately most of my longer shots are over pre-measured distances so have managed without so far .
Juggling available funds between too many sports at the moment .Perhaps when I sell off that spare Leupold 6.5 - 20x ?

[ 08-26-2004: Message edited by: Aussie ]