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  • Writer's pictureJason Stanley

Tuning: Part 3 = Store Bought Ammo

Updated: Mar 20

It is safe to say that the majority of hunters buy their ammunition preloaded from a manufacturer. The manufacturer is the one who controls the components used in the cartridge. This implies that hunters who buy ammo have no control over the tune of their rifle. they?

Here is the procedure for tuning a rifle using factory ammo.

1. Shoot Barrel into the Precision Window

2. Research ammo

3. Purchase ammo

4. Shoot groups

It is very difficult to make informed decisions without reliable data. Tuning is no exception. The majority of shooters know to try different ammo if the current lot does not group well. However, there are some key points in each step that need to be mentioned if you want to have reliable data. The attention is in the details. If you want to get the best tune using factory ammo then read on.

Step 1: Shoot Barrel into the Precision Window

It is a waste of time and money trying to tune a rifle with a barrel that is not in its accuracy window. If the barrel is too dirty, that data will be unreliable, which is worse than no data at all.

On the opposite end of the spectrum is a clean barrel. To get a clean barrel into the precision window a fouling shot (or two) may be needed. For a more detailed explanation of the precision window, see the Blog titled Cleaning: Part 1 = Know When to Clean.

Bottom line: Do all of the testing while the barrel is in the precision window. The easiest way to make sure that happens is to clean the barrel then take at least one fouling shot before you start shooting groups.

Step 2: Research Appropriate Bullet Weight

There are ranges of optimal bullet weights for each game animal. There may not be enough penetration if you shoot too light of a bullet. Shooting too heavy of a bullet and there will not be the desired expansion and most of the energy will simply pass right through the animal. Do your research and select the proper weight bullet for the animal you are pursuing.

Twist Rates

Different weights of bullets may shoot tighter groups than other weights. This could be due to the twist rate of the rifle. Without making this too confusing; bullet length, bullet diameter, and bullet weight all play roles in this equation. Manufacturers list the bullet weight and diameter but very seldom their bullet length. There are several on-line twist rate charts that attempt to match the twist rate to bullet weight. Some ammo/bullet manufacturers are finally publishing the ideal twist rate for a particular bullet.

To find the twist rate of your barrel; insert a cleaning rod with a snug fitting, cotton patch wrapped around a nylon brush into the chamber. Mark the location of the rod at the chamber with a perpendicular line on the rod and a parallel line by the handle. Slowly, insert the cleaning rod, watching the parallel line at the handle. Once that line makes one complete revolution, mark the new location of the rod at the chamber. Withdraw the cleaning rod. Measure the distance between the two chamber marks. This will be your twist rate. Example: one complete revolution in nine inches = 1:9 twist rate.

Does this mean you can not shoot the proper weight bullet if it does not match up with the twist rate of your rifle? Not exactly. The downside could be in your accuracy/precision. You will have to be the judge if the level of precision is acceptable.

Step 3: Purchase Ammo

Purchase different brands of ammunition whose bullet weight correlates with what is needed. To make this step more economical, partner up with some shooting friends. Four rounds will be needed for each brand chosen. A box of twenty could financially be split five ways. That makes tuning a lot more affordable.

There are many variables that can affect the tune of the rifle; shape, length and weight of bullet, type and amount of powder used, and length of the cartridge (C.O.L & B.O.L.). The downside to buying factory ammo is that we are at the mercy of the manufacturer and what they chose to use at the time the cartridge was made. More on this important fact later.

C.O.L (Cartridge Overall Length). This measurement is taken from the base of the case to the tip of the bullet. I use a caliper when taking this measurement. The only time I use this measurement is finding my maximum length due to magazine size.

B.O.L (Base to Ogive Length). This measurement is taken from the base of the case to the ogive of the bullet. I use the Hornady Lock-N-Load Bullet Comparator attached to a caliper to get repeatable measurements. This is the measurement that I use when keeping track of overall length in relationship to tuning.

Step 4: Shooting Groups

If we want reliable data, it is imperative that we keep as many variables out of the equation as possible. In other words; it is important to compare apples to apples. For example; Day 1 you shoot Brand X ammunition over a solid rest on a cement bench while using wind flags. On Day 2, you shoot Brand Y ammunition over the hood of your truck. There are simply too many variables at play to say that Brand X is better than Brand Y.

Variables that can change Variables that should stay constant

Bullet Weight Precision Window

Manufacturer Rifle Stability

Barrel Temperature


Distance to Target

Air Temperature

As mentioned above, keeping the barrel in the precision window is one of those variables that needs to be controlled. Another variable is how stable the rifle is held. I realize not everyone has access to a cement bench, but a solid platform is needed. The temperature of the barrel is another variable that you want to keep an eye on. A barrel warm to the touch is OK, but it should not feel hot. Once you get to that point, it is time to let the barrel cool if you want reliable data.

A major set of variables are the conditions you shoot in. The fun thing is that you can not control the wind or mirage. (minus the select few who have wind tunnels) You can only recognize the effects. I like using wind flags when tuning and practicing. However, any wind indicator will work. Use the wind indicators to find the day(s) with a light wind to give you the best data for your tune.

I strongly believe the worst condition to tune in is heavy mirage. Shooting in a heavy mirage is like shooting through a fishbowl. Very few shooters can obtain accurate data in those types of conditions. The second worst condition is no wind (usually early in the morning or late afternoon). These calm periods are when the “gremlins” come out. Some groups may be great, others may be horrible. Light wind is actually needed to even out those “gremlins” to give you a true idea of the tune.

Another variable you can control is the distance you shoot. I prefer to tune at one hundred yards and then confirm at two hundred. Most of my hunting shots are taken under two hundred yards. If you shoot at longer distances, then you may consider tuning at two hundred and confirming at three hundred. Tuning past three hundred is very difficult because the conditions play such a huge role on where each shot lands. This makes it tough to know if the group was caused by tuning or the wind.

I also like to tune on days with a temperature that will decently (+/- 30 oF) mimic the temperature in which I will be hunting. Temperature does play a role in tuning. However, I am not sure a factory rifle with factory ammunition would show that. Regardless, the closer the temperatures match up, the more reliable the data will be.

Most shooters have jobs, family, other responsibilities. The reality is, due to a lack of time, we sometimes have to shoot regardless of the conditions. When the shooting happens on "non-ideal" days, simply make note of that on the targets. That will help gauge the validity of the data when comparing different ammunition.

Here is the target I use for tuning factory loads. (Found in Reproducible) One brand of ammunition per target. Four rounds per brand. One round is for a sighter shot, just to make sure the rifle is printing on paper, and three rounds for the group. The best group wins.


We want the groups to be as small as possible. However, there comes a time when we have to say; ‘this is good enough’. Once that point is reached, our success in the field is not going to depend on the tune of the rifle, but on all the other factors that go into making the shot. The “good enough” point means it is time to quit tuning and start practicing.

Only you can determine what your “good enough” mark is. Each individual will have their own set of variables that go into determining this point. Think about the application of the rifle and come up with realistic expectations.

Example Factory Ammo Testing

Nosler 90 gr Trophy Grade .243

3.312" 3 shot group @ 100 yds

Winchester 100 gr Power Point .243

2.064" 3 shot group @ 100 yds

Nosler 90 gr Ballistic Tip .243

2.867" 3 shot group @ 100 yds

Analysis of Targets

1. None of these groups are acceptable. I need to keep searching.

(I will post updates when I find what shoots acceptable in this rifle)

2. All indications point to more powder, which makes sense. The

manufacturers need to make sure their ammunition is safe in all firearms.

3. The Winchester brand is the winner so far. The hunting shots just have to be within 150 yds.

4. Groups like this are the main reason I reload.

5. A two inch rifle still kills deer.

Now what?

The bar is set. No matter how big or small the groups are, now there is a baseline. We either keep searching to get smaller groups or we are satisfied that the smallest group is “good enough”.

If the groups are not acceptable (as in the above example) then keep searching. Try different brands of ammunition. Experiment with different weights of bullets. Another option would be to pay a hand-loader to develop a load for your rifle and reload the cartridges for you.

Experience Tip: Refrain from saying anything derogatory about the ammo tested. Just because it did not shoot well in your rifle does not mean it is "bad" or "does not shoot". That particular ammo might shoot great in a different rifle. The idea is to find what your rifle likes!

Once the groups are “good enough” return to the same store and purchase a couple boxes. (No, I am not promoting hoarding) Remember what was written earlier; We are at the mercy of the manufacturer and what they chose to use at the time the cartridge was made. Manufacturers can change their components. This will change the tune of our rifles. Do not assume that buying the same brand of ammunition this season will shoot the same as last season’s run.

The attention is in the details. Be mindful of the variables that can be controlled so you can obtain reliable data. You may be surprised at how much precision can be gained when tuning with factory ammo. Until next time, enjoy the process.


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