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Discussion Starter #1
Are there different ways to calculate group size? What about average group size, # of shots per group, if range is not given do we assume 100Yds for rifle and 25yds for pistol and 50 yds for Single shot pistol? I wanted to compare some 22 manufactured loads and realized I really did not know how to measure groups.
 

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Hi, Chief:
  Good question. Most people measure groups by going centre to centre on the two shots that are farthest apart. This is why benchresters list groups smaller than the bullet size.

  Most rifle groups are at 100 yards, unless specified otherwise. I don't know about pistols. Our indoor range distance is 20 yards, but outdoors??

  The average 10 shot group will be 1.7X the size of a 5 shot group, IIRC. It might take me a while to dig up the reference for that one, like next all of next winter.

  If conditions aren't specified, you may be comparing apples and oranges.

Bye
Jack



(Edited by Jack Monteith at 10:47 pm on June 2, 2001)
 

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Discussion Starter #3
So if you shot a 5 shot group but you know one was a flier because the shooter made a noticable error, you would measure the two furthest away of the 4 better ones if accuracy of the gun was what you were looking at?
 

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Hi, Chief:
  Yes, if you are sure it's shooter error. However, be sure the gun isn't stringing (the group is consistently oblong). It usually takes 3 or 4 groups before you're sure. A spotting scope is very useful to confirm that the shot that felt bad was the one that went out.

  Changing ammo often cures stringing. If you're shooting a typical bolt action with fore & aft action screws, be sure the screws are tight. If it's still stringing, loosen the rear screw to about half the torque of the front one and try it.

  Both Mic McPherson and Karl Bosseman (sp?) recommend loosening the fore end of a lever action if it isn't shooting.  If that does it, file a little wood off where the fore end is binding.

  Only measuring extreme spread on a 10 shot group means you're wasting the data of 8 shots. Measuring mean radius takes longer, but accounts for all the shots.

Bye
Jack
 

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Discussion Starter #5
There's a new one. Mean radius. What's the formula for that one? I do get the stringing( usually going up) . Is this a beading problem? Does does it occur as the barrel heats up or just from harmonics of the barrel? Man I sound like I know something but I don't. I promise.
 

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Hi, Chief:
  If the point of impact moves in one direction as you shoot, it's likely a heating problem. Try a couple of quick shots to warm it up, then wait 5 minutes between shots, so the barrel is at the same temperature for each shot. That should tell you if the barrel is warping.

  Some barrels will always throw the first shot through a clean barrel out of the group. Others will throw the first shot from a cold fouled barrel out. Some will throw the first shot out no matter what you do. A few will put the first shot in the group, clean or fouled. If you have one of those, treasure it.

  "Harmonics" means the barrel is whipping around like a loose garden hose under pressure. You can't stop vertical whip but you can find a load that exits the barrel when it's at the end of the whip and slows while it's reversing direction, with a bit of luck.

  Sometimes changing the rear action screw tension does the trick. This is most effective with barrels that rest on a pressure point at the front of the stock.  I don't know if it's worth trying if you've got a pillar bedded action and a free floating barrel.

  If the vertical spread isn't a heating problem, it's likely a harmonics problem. If the stringing is off vertical, it's likely a bedding problem and that's a whole 'nuther topic.

  I'll quote General Hatcher on mean radius, since he was a better writer than I am.

  "Mean Radius is the average distance of all the shots from the center of the group. It is usually about one third the group diameter."
  "To obtain the mean radius of a shot group, measure the heights of all shots above an arbitrarily chosen horizontal line. Average these measurements. The result is the height of the center of the group above the chosen line. Then in the same way get the horizontal distance of the center from some vertical line, such as for instance the left edge of the target. These two measurements will locate the group center"
  "Now measure the distance of each shot from this center. The average of these measurements is the Mean Radius."

Bye
Jack
 
 

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Jack,

Here's one to ponder, my own Marlin 444P. With hunting guns I shoot 3-shot groups. I figure beyond that and my game will either be long gone or laying on the ground. Anyhow, this particular carbine will shoot the first two almost touching and throw the third out as far as 2" at times. Now here's the kicker, it matters not if the barrel is warm or cool. I've let five to ten minutes pass between shots and she still does it every time. Load intensity doesn't affect it either. Kind of odd, wouldn't you say?




(Edited by Bill Lester at 1:57 pm on June 3, 2001)
 

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Hi, Bill:
  I can't figure out why Marlins shoot as well as they do, with all that stuff hanging on the barrel. My old .35 Remington has a barrel that Galliard, Hart, Lilja, Shilen et al. would trash. It has swarf rings all the way down the bore, but 90% of my 5 shot 100 yard groups stay under an inch and a half.

   OK, here's a bunch of wild guesses. Obviously, the heat's moving something, but what? :confused: I'd try loosening the fore end cap and trying that first. If the wood is sticking at both ends, then slips when the barrel expands and sticks again; that would explain why your third shot stays out after 10 minutes cooling. You'd have to let the barrel cool completely before the wood slips back again.

  If the barrel hasn't been stress relieved, it may be warping as it heats up. Elmer Keith claimed a straightened barrel was pure poison because it would walk it's shots as it warmed up. How should I put this? Suppose one side of the barrel is under more tension than the other, due to straightening. You fire two shots, and the tensioned side stretches more, but not enough to change the point of impact. Now, the bore surface is hot, but the outside of the barrel is still relatively cool. Now if I'm on the right track, here's the tricky part. Metal on the inside of the bore doesn't have as much leverage as the metal on the outside of the barrel. The heat has enough time to migrate to the outside of the barrel and warp the barrel by the time you fire the third shot. Does that make sense???

  Your barrel might be a candidate for the cyro treatment.

Bye
Jack  
 

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Paco Kelly wrote about how to accurize lever actions a number of years ago, in the fouling shot. I'll need to dig those article up because I now own two lever actions.
It's my experience that the larger the caliber the more consistant the groups. I've owned a Siamese Mauser for 25 years that was converted to 45-70 by Scotty in Killeen Texas. His workmanship was less then ideal, but that darned rifle will put most any bullet, then the weight range between 405 and 500 grain cast bullet into the same hole, using a fairly heavy load of IMR4064 or IMR 4320. The location of the groups will only be a inch or so different from the next bullet weight.
I'm now tinkering with a Winchester 1886 in 45-70 using a 350 grain cast slug. The groups, as the powder charge goes up, have not changed location very much, they just get a bit smaller.
I use the 5 shot group as a guage. But I will fire four or five groups of 5 with cleaning in between.
Are the larger calibers less prone to barrel whip/harmonics, then the smaller calibers?
Jim
 
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