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Thread: Measuring your groups...

  1. #1

    Measuring your groups...

    OK,

    So im looking for the DEFINITIVE answer on how to measure your group sizes, as per a target shooter for a competition. As i've never shot competitively with a rifle or been anywhere near a rifle club, i don't know if my way is correct.

    I look at the holes and figure what the smallest diameter circle all of those shots will fit within, thats what i say my group is.. Reason i ask is because when you get sub MOA, its more tricky and when people say they're getting 1/4'' groups, i don't know how i would measure it accurately. Especially when your bullets are in my case .300'' diameter.. I know that the deer doesn't care, but i do.. Im now getting decent groups which are within my thumbnail at 118yds, but trying to figure out the measurement accurately, i'm not sure how..

  2. #2
    Simple, measure the extreem spread of the group, outside to outside then deduct the bullet diameter.

    My only advice is, be honest with yourself, (and us if you are bragging!) Shoot five shot groups and measure the distance across the furthest apart holes.

    It really bugs me when I hear all the old chestnuts about a two shot group, a really good group if you discount the one I pulled four inches left, I never count the first shot as that is always high, the first two went high so I aimed down a bit and fired the rest of the group, need to fire a few fouling shots before she will group properly, there was a 1mph crosswind that pulled that round, etc etc

    Trust me, a true half MOA rifle/scope/ammo/firer combination in a sporting rifle is a very rare thing despite what you will read on the internet!

    JC

  3. #3
    It's the measurement from the centers of the two farthest bullet holes. If you're shooting a .30 caliber with .308" bullets, measure to the outside edges of the two farthest spaced holes and subtract .308" that will be the group size. In the old days of target shooting they measured "string length" which was the distance from the center of the target to the center of each bullet hole. These measurements were totaled and that was the measurement of the group.~Muir

  4. #4
    To save having to do any arythmetic, just measure from the inside edge to the outside edge of the two furthest spaced bullet holes. This has exactly the same effect as subtracting the bullet diameter, giving the "centre to centre" measurement.
    You can't say muntjac without saying, Mmmmmm.

  5. #5
    Cheers guys, JC275, you made me chuckle with the excuses people make! Ive never been a 5 shot group man.. I was always taught 3 shot groups due to barrel heating and you should be able to get a good feeling of the POI from 3 shots.

    What do you say regarding barrel warping after 5 shots, does this give a true account of the rifles accuracy?

  6. #6
    [QUOTE=flyingfisherman;129030]Cheers guys, JC275, you made me chuckle with the excuses people make! Ive never been a 5 shot group man.. I was always taught 3 shot groups due to barrel heating and you should be able to get a good feeling of the POI from 3 shots.

    What do you say regarding barrel warping after 5 shots, does this give a true account of the rifles accuracy?[/QUOTE

    Although I can't deny that barrel heating can and does have an effect, it is often just another one of those excuses. I have shot service rifle competitions where you fire upto 80 rnds in about six or seven minutes and still turn in winning scores. If barrel heating is having a significant effect on your rifle during a 5 shot group then I would question whether it is too sensitive for the rigours of stalking?

    JC

  7. #7
    ff. I downloaded the following:
    OnTarget Software - seemed a bit complicated so I deleted it. Found the following link today:
    http://rawilson.net/shareware/index.html looks a bit better, but I haven't tried it yet.
    Rgds JCS
    Last edited by jcampbellsmith; 23-08-2010 at 17:35. Reason: clarification

  8. #8
    ff. Update - My old school ruler knocks spots off both pieces of software. I would be interested to see if anyone can find a worthwhile scanning program. Rgds JCS

  9. #9
    Cheers JCS.

    JC275 - RE barrel warping, lighter hunting barrels heat up quickly and the groups expand too, im sure i dont need to tell you that, but the addage that a stalking barrel will be unlikely to fire 3 shots one after another on a stalking outing so heat dissipation isnt as important with a stalking rifle as it would be with a target machine..

    Do 3 shot groups not exist in the competitive world of shooting? I shall start doing more 5 shot groups and see what i come out with.

    Also, if i want to work out my group as an MOA, what formula would i use? obviously its easy at 100m/200m/400m etc but at odd ranges such as 118yds (how long the place is where i zero my rifles).. Is it a simple trig formula? would it be T=O/A? or am i over complicating it?

    PS: Ive found an MOA calculator and have been lazy. My .308 groups at 0.8 MOA.
    Last edited by flyingfisherman; 23-08-2010 at 18:57.

  10. #10
    Quote Originally Posted by flyingfisherman View Post
    .... Is it a simple trig formula? would it be T=O/A? or am i over complicating it?
    ff
    Try 2 x 3.14159 x the range in yds x 36 divided by 360 divided by 60 (2 pi r for circumference, 36 to convert yds to inches divided by 360 degrees divided by 60 minutes)

    Therefore 1 MOA measures about 1.24 inches at 118 yds.
    Anything, but anything is more interesting than weekly project reports. Rgds JCS

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