Can someone point me to a source that gives the conversion factors to convert 3-shot group sizes to 5- and 10-shot group sizes? In other words, if I have an average 3-shot group size for a rifle and load, what is the conversion factor to estimate what the 5- and 10-shot group sizes would be with the same rifle and load?
Converting 3-shot group size to 5- and 10-shot group size
- Thread starter South Pender
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drop_point
Silver $$ Contributor
Impossible to predict, the three rounds are statistically insignificant and there are factors unique to each rifle such as barrel diameter, bedding, etc.
For 3-shot vs 5-shot, X 1.4 will get you close. 
USE an average, not a best/worst . . . to obtain average, the more you shoot, the closer you'll get . . . but how much wear do you want/need to erode the barrel? 
RG
USE an average, not a best/worst . . . to obtain average, the more you shoot, the closer you'll get . . . but how much wear do you want/need to erode the barrel? RGMultiply a 3 shot group by 1.23 to get what you would expect for a 5 shot group for a long run average. There are many references such as Creighton Audette "Testing Rifles and Ammunition." Also multiply a 5 shot group by 1.24 to get a 10 shot group or multiply a 10 shot group by 1.17 to get a 20 shot group.
Yes, that's what I'm looking for. I have read 1.28 somewhere for the 3 to 5 conversion. Does Bryan Litz, perhaps, have a formula? If so, where can it be found?
The Audete reference is based on Dr Frank E. Grubbs, "Statistical Measures of Accuracy for Riflemen and Missile Engineers" and that is also the source for other references. I have the Litz books and I will see if I can find his reference.
pdog06
Gold $$ Contributor
I have to ask. Why does one need to convert a group size from a 3 shot group to a 5,10,20 shot group, without physically shooting those groups?
I know that if I stopped at 3 shot groups instead of 5 or 10 my average group size shrinks a lot! It’s usually shots 4 or 5 when I screw up…lol
Not trying to be a smartass at all. Sure there’s a valid reason. Just curious why and what this estimated data is used for given the fact that there are so many variables?
I know that if I stopped at 3 shot groups instead of 5 or 10 my average group size shrinks a lot! It’s usually shots 4 or 5 when I screw up…lol
Not trying to be a smartass at all. Sure there’s a valid reason. Just curious why and what this estimated data is used for given the fact that there are so many variables?
Yeah, me too. I don’t care what the multiplication factors come out to be.
Means nothing.
If it meant anything to me I have some 3 shot groups during my whole life that are world champions.
I’ve had many really great three shot groups that seem to be destroyed by the 4th or 5th shot. I shoot against myself. And that last shot or two the pressure is on not to wreck it. I tell myself ‘no pressure’ but that’s really BS
Means nothing.
If it meant anything to me I have some 3 shot groups during my whole life that are world champions.
I’ve had many really great three shot groups that seem to be destroyed by the 4th or 5th shot. I shoot against myself. And that last shot or two the pressure is on not to wreck it. I tell myself ‘no pressure’ but that’s really BS
Yeah - old age caught up with me . . . back-in-the-day, many [excellent statistical] articles were published in PRECISION SHOOTING MAGAZINE!
The 1.4 factor is what I've used for thirty-five+ years - it was probably a close approximation for 3 to 10-shots.
The reason, is to reduce "wasting" time on loads producing and average of, say, 0.2" [100 Yd.] three-shot groups, and fooling oneself into thinking it's a "competitive" load - three-shot groups need to AVERAGE <0.15".
I usually just shoot five and ten-shot groups, and get it over with . . . RG
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Choot em man! Formulas and calculations are great but believe the target above all else. Stuff is always gonna be expensive and yes, worse now than ever...but we've always complained about it and I see no reason for that to change, in the foreseeable future. The number one thing you can do to help your shooting is to shoot more. And I mean GOOD practice. That IS the only shortcut to getting better IME. It really boils down to wanting it more that the next guy. No other way to put it and shortcutting group size by calculating probabilities will not make you a better shooter. The thing about this game is simple...you buy match wins at the practice bench. Shooting more and treating every shot during practice just liked a match..IS the shortcut. Ya gotta want it more than the guy sitting next to you. I hope my fellow competitors spend their time on calculations while I'm shooting. If that happens, I'll win a lot. One problem with that is that every top shooter already knows it!
pdog06
Gold $$ Contributor
Thanks for the clarification. I figured it had something to do with just saving barrel life and supplies.
Personally, for my own satisfaction, I feel that a 3 shoot group is just too small of a sample size, and if you’re gonna shot a bunch of 3 shot groups to get an average, why not just shoot 5 or 10 shot groups to begin with and just shoot less groups. I will believe it when I see it on the targets.
Personally, for my own satisfaction, I feel that a 3 shoot group is just too small of a sample size, and if you’re gonna shot a bunch of 3 shot groups to get an average, why not just shoot 5 or 10 shot groups to begin with and just shoot less groups. I will believe it when I see it on the targets.
As others have implied, trying to extrapolate a 3 shot group to a 5 / 10 shot group is pure folly. You're deluding yourself if you believe you can reliability predict dispersion with statistically insignificant sample sizes.
Even though larger samples sizes will give you better data, there is the aspect of the dynamics of shooting, it is not a static system meaning each shot can be different depending on the amount of fouling being laid down, barrel temperature, variation is components, and most of all the human element.
In my experience, the best that can be achieved is data which over time will reveal, on average, the best load for that rifle.
Even though larger samples sizes will give you better data, there is the aspect of the dynamics of shooting, it is not a static system meaning each shot can be different depending on the amount of fouling being laid down, barrel temperature, variation is components, and most of all the human element.
In my experience, the best that can be achieved is data which over time will reveal, on average, the best load for that rifle.
...yes, and tune changes throughout condition changes. Sometimes more than others, so the days it holds pretty steady, you "might" learn something from his proposal but the fact is, physics just get in the way and there is no tune that stays perfect, all of the time. So you must adjust or the data is flawed.
I discuss these facets with customers almost daily. For those to whom preserving barrel-life and components is improtant, three-shot groups CAN be useful in identifying where more incremental tuning may be worthwhile: identifying both where to begin,"splitting the [tuning] frog-hairs", and their ability to deliver.
Scanning for powder charge, seating-depth, etc., with 3-shots, I always agree, is ok. However, confirming with multiple 5 and/or 10-shot groups is desirable . . . then, participating in a few tournaments, will identify what one has - or, hasn't got.
RG3 shot bracketing potential loads is something I have done a lot. It can help refine and narrow selection. However, before going into full scale production of a load, e.g., for varmint season, I verify a load recipe with several five shot groups.I discuss these facets with customers almost daily. For those to whom preserving barrel-life and components is improtant, three-shot groups CAN be useful in identifying where more incremental tuning may be worthwhile: identifying both where to begin,"splitting the [tuning] frog-hairs", and their ability to deliver.
Scanning for powder charge, seating-depth, etc., with 3-shots, I always agree, is ok. However, confirming with multiple 5 and/or 10-shot groups is desirable . . . then, participating in a few tournaments, will identify what one has - or, hasn't got.
With today's shortages and prices, careful selection of potential loads and subsequent testing needs to be efficient and economical. I found that a good place to start is with the accuracy load published by the bullet manufacturer. With few in any exception, I found these loads with some minor tuning of powder charge or seating depth to be quite effective in finding a serviceable load.
Thanks for all your replies, guys. I wasn't advocating for 3-shot groups (or any particular size groups). My question was aimed more at being able to better understand reported range results. As just one example, what should one make of a reported average of 1.0" for a number of 3-shot groups? Does this point to a 1 MOA rifle? Since the 5-shot group average would be predicted to be something like 1.28", and the 10-shot group average would be predicted to be something like 1.60", the answer to me would be no. I'd just like to be better able to make sense of the accuracy results I come across from various sources.
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