Tuning: Trends and the Tune Window
Updated: May 2
Tuning is a systematic process where the shooter adjusts the reloading components (load) to achieve the best precision (smallest groups) possible. An abbreviated order of operations is to pick a powder and bullet - adjust the powder charge - adjust seating depth - go up/down in neck tension - then try different primers.
A frustrating aspect of tuning is that a rifle might be shooting “dots” one day then shoot “ugly” the next. One technique that will help keep that rifle in tune is to look for trends that point to a tune window while doing the initial load development. The wider the tune windows, the more forgiving the load.
Simply stated; a tune window is the range in the individual component being tested that shoots small. To find a window, look for trends in what the targets are showing when differences in the components are tested. The beauty of looking for trends is that one group does not make or break the trend. There should be several different indicators all pointing to the correct load.
Why should we find Tune Windows?
There are two reasons why we should find tune windows. First; conditions affect the tune of a rifle. A theoretical example: your favorite hunting rifle is tuned to shoot .75" groups on a day when the temperature is 70 degrees. However, if that same load is tested again during hunting season, when the temperature is 30, that rifle may not shoot .75" groups.
How can that be? Changes in temperature, humidity, pressure, and altitude can influence how the rifle groups. Temperature is the biggest piece of that puzzle. When the temperature changes, that can affect how the powder works. All powder is temperature sensitive, some are just more sensitive than others. Tune windows help us find a range of powder charges (and seating depths) that group well yet also give us the most forgiveness when the conditions change.
Second; we need a cushion in our reloading. There are many places we can have inconsistencies in our reloading procedures and/or imperfections in the products we use. A few examples: If you use a powder thrower, the way the handle is operated can influence how much powder is dispensed. If you weigh your charges, how accurate is the scale? Name brand bullet manufacturers may have multiple dies for the same bullet. This can cause the ogive to be in a slightly different location causing different seating depths. Tune windows will give us some "cushion" when working with these imperfections.
Experience Tip: In order to get accurate data for a tune window, the reloader can only change one variable at a time.
Powder Charge Trends and Tune Window
Here is a theoretical example of a trend and the tune window for powder charge. Notice that as the powder charge increased, the group size showed an inverted bell curve shape. Graphing the data makes the window much easier to see. The bottom of the bell curve was 34.4, 34.5, and 34.6 gr. That range of powder, where the groups were the smallest, is the tune window.
For the initial powder charge, pick the charge in the middle of the window. This will give the greatest “cushion” to account for differences in temperature, humidity, pressure, altitude, and reloading errors. In this theoretical example, the powder charge that I would focus on would be 34.5 gr.
Trends and the tune windows are easy to spot when the perfect “U” shape forms. However, outliers can make this a little more tricky. One of those is the “one hit wonder” group. The component that groups great, yet other groups around it are much larger. This single group takes the “smoothness” out of the data.
In this made-up example, the charge of 43.1 gr is very tempting, it shot the smallest group with a .092” However, the groups on either side of this were (relatively) larger. When the 43.1 gr. group is ignored, the smooth shape to the line returns. Remember; one group should not make or break the trend.
One of the repercussions of running with the 43.1 gr. powder charge is any change in the conditions could open up the groups. In other words, the tune window is not wide enough. For this example, the true window is 43.3 through 43.7 gr., with the sweet spot being 43.5
Another outlier to be careful of is the group I call the “one flop wonder”. Again, this group takes the “smoothness” out of the data, but instead of the group being smaller, it is larger. The “one flop wonder” does a good job of hiding the tune window.
In this example, the charge of 48.4 disrupts the classical “U” shape. Even though it is not as easy to see, the tune window is 48.2 to 48.5 gr. A plethora of things could cause this singular “bad” group.
Experience Tip: Looking for trends and the tune window is not the end of the tuning process. It is simply a tool to use during the tuning process. Charge weights (and seating depths) still need to be confirmed on paper in different conditions. This is especially true when dealing with “one flop wonders”.
Reality always trumps theory. Here are three targets that closely match the above theoretical examples.
First is a powder charge trend for a 1/18” twist Krieger barrel on my Hunter Class rifle. Notice that it is an almost perfect “V” shape, with the best charge at 34.4 gr. On this particular load development, I was increasing the charge by .2 gr instead of .1. The greater the difference in charge between groups, the more “V” shaped instead of “U” shape there will be to the data.
Here is the data for a 1/17” twist Krieger barrel on my Heavy Varmint Class rifle. This is a real life example of the “one flop wonder”. Interestingly, the arrows from this, and other, targets were all pointing to 34.7 gr even though it shot an ugly .34” group. I did pick the 34.7 gr but confirmed the load multiple times in multiple conditions. FYI: 34.7 gr is the powder charge I shot in this rifle at the 2021 Nationals in St. Louis. The rifle shot a 250-20X at 100 yds placing 5th. Just another example of trusting the trends, not individual groups.
Lastly, here is an example from a day of tuning my Remington 788 with a Krieger barrel chambered in 22-250. Even though the groups are larger than in the competition rifles, the data still shows a classical “V” shape.
Trends and Tune Window with Seating Depth
Before we begin, I would like to define some terms. This is especially important when discussing seating depth because not everyone uses the same definitions. The 0.000” mark is what I call the just touching mark. This is where the ogive of the bullet just comes in contact with the lands of the barrel. Jam would be when the bullet is seated long so the ogive “jams” into the rifling. This would be indicated with a + sign. For example; +.015 would be the bullet seated .015 longer than the 0.000” mark. Jump is when the bullet is off the lands and has to “jump” to get contact. This would be indicated with a - sign. For example; -.005 would be when the bullet is seated .005 shorter than the 0.000” mark.
Another term that should be defined is sticking point. This is the jam length that causes the bullet to stick in the lands when not fired and ejected. Stuck bullets can cause the powder to spill out in the chamber, possibly getting into the workings of the trigger. It is important to understand that the sticking point is based on how much neck tension is being used. In other words, the sticking point for one person might be different than another.
Seating depth trends work much the same way as powder charge, there should be an inverted bell curve shape to the data. Single groups should not make or break the trend. Again, we are looking for general trends pointing to a range of what the rifle wants. The wider the range, the more forgiving the seating depth is to the tune of the rifle.
The difference between seating depth and powder charge is where to pick the sweet spot out of the tune window. In powder charge, I like to go right in the middle of the window. That will give the greatest cushion for changes in conditions. For seating depth, I like to hedge on the long side of the window. As the barrel gets shot, the lands are going to erode. Starting on the long side of seating depth will give a larger cushion to account for the receding lands.
Experience Tip: For competition rifles, I do not like to go past +.020” jam because, to me, the risk of sticking a bullet outweighs the incremental gains in precision. However, I am not opposed to going further if that is the only place the rifle will shoot. For hunting rifles, I will not go past +.005 jam. In hunting rifles, the risk of sticking a bullet greatly outweighs any precision gained.
Neck Tension and Primer
Two other variables that can be tested and changed for the best tune are neck tension and primers. However, there simply is not enough data to look for trends or windows in those two variables.
I usually start load development with .003” neck tension for my competition rifles and .002” for hunting rifles. Once I find the powder charge and seating depth, I will go up and down .001” in neck tension to see if any improvements were made to the groups.
CCI BR 2/4 primers are my “go to primer” for all my initial tuning. Once I settle on all the other components, I will change the primers to Federal to see what happens. Rarely do I see a difference.
Competition vs. Hunting
Competition Tune Windows
To be competitive in the short range Benchrest, I need the rifles to tune in the high zeros to mid ones. (at 100 yds.) I will tune in the spring time before the first couple matches. I will then check that tune in the summer to see if any changes need to be made due to the increase in temperatures.
For the 30BR, H4198 seems to give me the best precision with the largest tune window. That is a win-win. When I do my job correctly and pick the right load, the tune window is large enough to "absorb" any temperature changes I will see at tournaments in the Midwest.
Not all caliber/powder combinations will do that. Other combinations may shoot extremely small groups yet have a very small tune window. This is the main reason many competitors reload at the range and/or have tuners on their barrels. These competitors need to keep their groups small, but because the tune window is so narrow, they will have to adjust their load as the conditions change.
Some competitors do not have the luxury of reloading at the tournament. Therefore, it is important for them to find a larger tune window to keep the needed precision when the conditions change.
Hunting Tune Windows
Hunting rifles do not require the utmost in precision like a competition rifle. When I can get my hunting rifles to group around a half inch that will be good enough. (Obviously, I will go smaller if I can) However, I am looking for a large tune window along with that half inch.
I do most of my tuning, for hunting rifles, in the early to mid spring when the temperatures are in the 50's. However, when I hunt, the temperatures will vary from the low teens to mid fifties. I need a large tune window to compensate for this wide range in temperature.
The goal is to find a load that groups well and also has a wide tune window. That is a win/win situation. A good question is; what should I do in a win/lose situation? What do I do when my rifle groups great, but the tune window is not very wide? I believe you have two main options.
Option 1: Keep searching. Try different bullets, try new powders. Use proven resources to find powders that have wider tune windows, yet still provide the precision that is needed. Keep in mind you may have to sacrifice some precision for a larger tune window. Only you can decide if the trade-off is worth it.
For some people, the loss of precision is not acceptable. These shooters will need to go to option number two: adjust the load based on temperature. If you are a competition shooter and have the ability to load at the range, adjust your load based on the conditions. If you are a hunter and/or you do not have the ability to load at the range, you will need to keep accurate notes on where the sweet spot is in the tune window based on temperature. Then, adjust your load for upcoming temperatures.
An example of this is my .22-250. It groups great with a certain powder, yet the tune window is very small. I could have picked option 2; two different loads, one for summer and one for winter. I chose Option 1: I found a different powder that does not shoot quite as tight, yet has a much larger tuner window. In this particular case, the trade off was worth it.
A great "tool" to have in your "tuning tool box" is know how to look for trends that all point to a tune window. Be careful of outliers and remember that the sizes of the groups are all relative. Once the trend points to a particular load, always confirm that load in different conditions. Until next time, enjoy the experience.