It’s no secret that long-range accuracy is more difficult to achieve than short-range accuracy. Not only are mistakes amplified over the distance, but there are more factors at play. One of these factors is muzzle velocity.
When your longest shots are 400 or 500 yards, good groups at 100 yards can mean decent accuracy and shot placement. The guy shooting 500 yards or less may not even notice the effects that variable muzzle velocity have on accuracy. Beyond 500 yards, however, a wide variation in muzzle velocity will begin to affect the vertical dispersion of your shot string. The longer the shot, the wider the vertical dispersion. In other words, your shots will miss your aim point high or low, depending on the variation in muzzle velocity.
There are a few terms that you need to fully understand here. The first is Muzzle Velocity (MV). The concept is simple. Muzzle velocity is the velocity of the bullet measured at 15 feet from the muzzle. The second term is Extreme Spread (ES). Extreme spread defines the difference between the highest and lowest numbers of a group. For example, the extreme spread for a velocity string of 3000, 3010, 2990, and 3025 would be 35 fps. Using a chronograph, an instrument that costs roughly $100 to $250, you can measure the MV of a group of shots, then determine the ES by subtracting the lowest MV number from the highest.
The effects of a high spread of muzzle velocity can be devastating to your overall accuracy. A spread of 65 isn’t uncommon in factory ammo, and we’ve seen numbers as high as 150 in premium ammo that runs about $5 a shot! You may not notice a difference at 100 or even 400 yards, but at 800 yards, an ES of 65 can mean 6.5 inches of vertical dispersion on your target, and these numbers increase dramatically with more distance. With a 7mm cartridge and a 168 grain VLD bullet, an ES of 30 fps will cause about 5” of vertical dispersion to your group at 1000 yards. For this reason, we consider 30 fps the guideline maximum extreme spread on a long-range hunting system.
So, what do you need to do to achieve a 30 fps maximum extreme spread? Non-loaders will have to rely on testing different brands of ammo. Hand-loaders have more control over their ammunition, so you can make adjustments that will affect your ES. The elements that will have the greatest effect on MV are powder and primer selection.
The first thing to consider is powder selection. Selecting the right powder can be tedious — it’s often just trial and error and involves hundreds of loads. One of the factors that affects powder is temperature. It’s important to find a powder that remains consistent in any temperature where you may be hunting, whether it’s 90 degrees in July or zero degrees in January. Keep in mind, just because a certain powder isn’t listed as an extreme powder doesn’t mean that it won’t behave properly, it just means that you need to test it.
The test is simple. Head out to the range on a hot summer day with 10 loads of shots — 5 of them in a cooler, chilled. Compare the extreme spread of the loads of cold ammo versus the loads of hot ammo — that will give you an indication of how the powder performs at different temperatures.
Once you’ve narrowed your powder to a few choices that remain consistent over a wide temperature range, you should look at chamber pressure. Some powders will call for loads approaching the SAMMI maximums. The main lesson is to not to give up on a great powder just because your starting loads don’t show promise.
With regard to maximum loads, the only way to measure the pressure of a cartridge is to monitor the muzzle velocity with a quality chronograph. Different chambers and bore dimensions will have different pressures, and no two rifles, even from the same manufacturer, are exactly the same. You may get lucky just dumping the maximum powder charge listed in a load manual. Then again, the pressure may be too low or too high.
It’s important to note that the load manual’s suggested muzzle velocity for a certain powder should be considered the maximum velocity. The best thing to do is to use data from several manuals to find an average maximum velocity for the cartridge and powder and load to that velocity. Often, the maximum load listed will actually produce an MZ that’s hundreds of feet slower. By loading to the velocity, it’s possible to achieve a reasonable velocity with running high pressure. Pressure and velocity are related, so you can use measurements of one to monitor the other.
There are a couple other elements that affect long range accuracy. One is primer selection. When changing primers, it’s best to reduce your powder charges at first, then gradually return to your maximum velocity in gradual increments. Unfortunately, the only way to determine which primer is the best one to use is through trial and error, which can be a tedious process. The wrong primer can make a good powder look bad, so choosing the right combination can be tricky — be careful when drawing conclusions.
Another factor to consider during load development is brass preparation. The two main factors that affect load performance are case capacity and neck tension. A general rule of thumb for a large rifle case is +/-7 grains of brass weight can mean nearly 1 grain of powder (plus or minus) in case capacity. That 1 grain difference can cause a difference in velocity of 50 fps — well outside of our 30 fps maximum extreme spread on a long-range hunting system. Neck tension can be determined by examining the dimensions of the neck diameter after sizing. After seating the bullet, the difference should be between 0.001 and 0.002 inches.
To achieve a high level of long-range accuracy, you’ll have to deal with many issues that aren’t very important factors when shooting at distances of fewer than 500 yards. Often, your success as a long-range marksman depends entirely upon how much time and effort that you’ve put into preparation. We’ve all paid our dues. Eventually the preparation pays off, however, and you’ll be ready once the opportunity of a lifetime arrives.
Learn more at:
gunwerks.com| fb.com/gunwerks | youtube.com/user/gunwerks| instagram.com/gunwerks
