.....Bullet Selection for the .308 Winchester:
............Target Shooting Applications
.....The .308 Winchester is one of the most popular target rifle cartridges in the world. There are several well known mid and long range target shooting disciplines that center around the .308 Winchester cartridge including Palma shooting and certain divisions of F-class and Service rifle matches. In order to provide the best and most properly tailored information about which Berger bullet to use for the .308 Winchester in Target shooting applications, I’ll present a ballistic analysis that is representative of two different types of rifles. The first is a representative Palma rifle which will be similar to an F/TR platform in terms of achievable ballistics and recoil. The second will be a service rifle. The differences between these two rifle types that are important to a ballistics analysis are rifle weight, barrel length, and max chamber pressure. The weight of the rifle affects the felt recoil for a given load and the chamber pressure and barrel length dictates the muzzle velocity that can be achieved.
Before looking into the ballistics of the different bullets available for the .308 Winchester, you should consider the type of shooting you’re intending to do because your bullet selection may be dictated by the rules. For example, if your goal is international Palma or Fullbore competition, you will have to use a bullet weighing less than 156 grains. Also, if you’re shooting a .308 as an F/TR rifle, you may also be limited to bullets weighing under 156 grains. In those two cases you’re spared the burden of decision making. However, most Fullbore and Palma matches that are fired in the US including individual and team events have no restrictions on bullet weight.
The biggest uncertainty in long range target shooting is the wind. The best way to improve your score is to learn to read the wind better. However, you can also make smart decisions about your equipment that can minimize the effects of wind, thus increasing your margin for error. This is where bullet selection comes in. As we’ll see in the following analysis, you can decrease wind deflection by using heavier, higher BC bullets. Nothing in this world is free, and the price of this improved performance is added recoil. The following analysis will give you an idea of the relative wind deflection and recoil for the Berger Target bullets recommended for Palma and Fullbore Target shooting.
The following analysis is based on a max chamber pressure of 60,000 psi. This is the approximate pressure required to propel 155 grain bullets to a muzzle velocity of 3000 fps which is a benchmark velocity for this bullet and application. The muzzle velocity for heavier bullets is based on the same 60,000 psi of chamber pressure. Obviously the muzzle velocity is less for heavier bullets for the same chamber pressure.
Figure 1 shows a typical modern Palma rifle. A rifle weight of 13 lb is used for the recoil calculations and a barrel length of 30” is assumed for the muzzle velocity assessments. The information in Figure 1 will be used as the initial conditions for the following ballistic performance assessments. A note about recoil: Figure 1 shows the recoil of the rifle in ft-lb. Most people aren’t familiar with gauging felt recoil with a number of ft-lb. The recoil is presented as a comparative measure. In other words, if you know how the recoil of your Palma rifle feels shooting 155 grain bullets at 3000 fps, you can see from Figure 1 how much more recoil you’ll feel by going to a heavier bullet. For example, the 210 grain bullet produces 7.1 ft-lb of recoil compared to the 5.3 ft-lb of the 155 grain bullet. That’s 34% more recoil for the heavier bullet, and you can get an idea of how the recoil of the heavier bullet would feel (34% harder).
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Wind Deflection
The debate about bullet weight, BC, and muzzle velocity as it pertains to wind deflection is a complicated matter because of all the variables that are usually involved. However, since the current analysis is only examining the performance of one particular case (.308 Winchester) it’s quite easy to demonstrate that the heavier bullets perform better in the wind compared to lighter bullets when both are fired from the same rifle with the same chamber pressure. Of course the heavier bullet starts out slower, but is able to retain its velocity better and will usually be going faster than the lighter bullet at long range which will be discussed in the next section on velocity.
Figure 2 shows a comparison of wind deflection for Berger bullets. The wind deflection is calculated for a 10 mph crosswind at 1000 yards in standard atmospheric conditions. Remember it’s not the specific amount of wind drift for a particular bullet that’s important; it’s the relative comparison between them. For example, consider the 155 grain VLD with 100” of deflection compared to the 185 grain VLD which has only 83” of deflection in the same conditions. In this case, the heavier bullet has 17” less wind deflection in the 10 mph crosswind. If the crosswind were only 5 mph, each bullet would have half the wind deflection shown in Table 1; 50” for the 155 and 43.5” for the 185.
Notice the wind deflection is not completely determined by bullet weight and velocity. There are at least two bullets available in most weights: a VLD and a non-VLD bullet. In most cases the two are comparable in ballistic performance. Both are made available to accommodate the various rifles and their accuracy preferences.
It’s clear that the heaviest (210 grain) bullets have the least wind deflection of all. Many people believe that 210 grains is too heavy for the .308 Winchester and that the cartridge is not capable of producing a high enough muzzle velocity to make this bullet perform well. This is a good lead in to the next section.
Velocity
The initial (muzzle) velocity is a common metric for ammunition. The fact that muzzle velocity is depressed for heavier bullets in a given chambering was addressed and quantified back in Figure 1.
Another important consideration involving velocity is retained velocity. In order to be a practically viable and reliable target load, a bullet must be traveling faster than the speed of sound when it reaches the target. Failing to achieve this will put the bullet at the mercy of the unpredictable effects of transonic aerodynamics and in some cases cause the bullets to lose stability at which point accuracy is out the window. The exact speed of sound varies with air temperature, but is around 1120 fps in standard conditions. The negative effects of transonic flight begin to build as the bullet slows down and spikes at the speed of sound. In other words, it’s a good idea to insure your bullets will remain well above 1120 fps at the target distance to avoid the onset of transonic problems. Aiming to stay above 1200 fps is good , and gives your bullets a little safety margin to avoid the worst of the transonic effects.
Table 2 shows some comparative metrics concerning retained velocity at 1000 yards which is typically the longest distance that target rifle matches are held. In addition to showing the retained velocity in fps and the percentage of muzzle velocity retained, Table 2 also shows how far beyond 1000 yards the bullet can fly before falling below 1200 fps.
You can see that all of the bullets remain above 1200 fps at 1000 yards when loaded to max pressure in a typical Palma rifle with a 30” barrel. The heavier (higher BC) bullets, despite being fired at a depressed muzzle velocity, reach 1000 yards with more retained velocity than the lighter bullets. Furthermore, the heavier bullets remain above 1200 fps for a greater distance than the lighter bullets that started out much faster. In some cases, the 308 is capable of keeping bullets above 1200 fps out to 1200 yards. Note the calculations shown in these tables are based on standard atmospheric conditions (59oF, 29.92 InHg, 0% RH). Exact results will vary depending on ambient air temperature, pressure, humidity and actual muzzle velocity.
Those who believe that a 210 grain bullet is too heavy to work in a .308 Winchester should study Tables 1 and 2 carefully. As long as the barrel has a fast enough twist and the shooter can tolerate the recoil, there’s no reason why a 210 grain bullet won’t work very well from a .308 Winchester.
Bullet Drop
Sometimes drop is mistakenly considered an important measure of merit by target shooters. The belief stems from hunting applications where range uncertainty is mitigated by a flat trajectory. In most target shooting applications, the range to the target is known and constant for each shot of a given string of fire. Some shooters mistakenly believe that a flatter trajectory means better overall ballistic performance, and that’s not necessarily true.
Bullet selection for the Modern Palma Rifle: Conclusion
If your bullet selection is not dictated by the rules of your particular discipline, then your choice basically comes down to weighing wind deflection against recoil. Of course some rifles seem to shoot certain bullet types more accurately than others so that’s certainly an important consideration as well.
In your efforts to minimize wind deflection, you shouldn’t overlook the adverse affects of recoil. Everyone has a different threshold for what’s a comfortable amount of recoil. If you exceed your limit you may lose more points because of poor shot execution than you can make up in superior ballistics. The effects of recoil go beyond personal comfort. Heavier bullets can affect the inherent accuracy of your shooting because they generate more recoil and spend more time in the barrel. You can offset the effects of elevated recoil by adding weight to the rifle, up to the limits dictated by your particular discipline.
As far as specific recommendations go, the Berger 155.5 grain FULLBORE bullet is an outstanding all around bullet to use in the modern Palma rifle. The low drag of the long ogive gives this bullet an exceptionally high BC for its weight. As a result, this bullet has great ballistic performance at a relatively low recoil level. You would have to go all the way up to a 185 grain bullet which generates 19% more recoil in order to beat the wind deflection of the 155.5 by a significant margin. In addition to the superior ballistic performance of this bullet, it has proven to be extremely accurate in most rifles and very forgiving over a range of seating depth. The first year this bullet was made available (2008) it was used to win the US National Palma Individual Championship with a score of 450-26X. It was also used to win the 2008 Midwest Palma Aggregate, and to set the US national record for the midrange course with a score of 450-39X.
The .308 Winchester in a Service Rifle
There are several rifles that are chambered in .308 Winchester (7.62X56mm NATO) and used in service rifle competition: the M14, the civilian version of that weapon the M1A, the M1 Garand, and the new M110. Figure 4 shows these 4 rifles along with their barrel lengths and weights.
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This is not intended to be a detailed assessment of the exact specifications of service rifles, but rather a focused look at bullet selection for the nominal 30 caliber service rifle. For the purposes of this ballistic assessment, we need to identify a barrel length and rifle weight that is representative of the class of service rifles used in target competition. According to the above metrics, a barrel length of 22” and a rifle weight of 10.5 lb will be used to baseline the muzzle velocity and recoil. The M1 Garand with its 24” barrel will be a little above the curve in terms of muzzle velocity, and the M110 with the 20” barrel will be a little slow. Also note the unusually high weight of the M110 will result in less recoil than shown for the nominal 10.5 lb service rifle. The trends involved in bullet selection will be the same for every rifle in this class, even if the exact numbers are a little different for each.
Another thing to note about this analysis is that it’s focused on long range performance because long range is where bullet selection is most critical. Bullets that are used for the shorter ranges and in the offhand/rapid fire events are more dictated by the shooters recoil tolerance than their external ballistic performance. In the case of short range events where bullet performance is less of an issue than recoil, Berger offers 5 lightweight flat base bullets to choose from between 110 and 150 grains. The remainder of this bullet selection analysis will focus on the external ballistic performance of the boat tail bullets used at mid to long range.
You’ll notice that the 210 grain bullets are missing from the list of service rifle bullets. Bullets that heavy are not recommended for use in the service rifles because of the pressures developed and the resulting damage that’s caused.
It’s interesting to note that the lighter weight of the service rifles combined with the lower muzzle velocities produces approximately the same recoil as the heavier Palma rifles that can shoot the bullets at higher velocity
Wind
Wind deflection is a challenge for service rifles, more so than rifles in other classes due to the shorter barrels and the compromised ballistic performance compared to dedicated target rifles. Table 5 shows the relative wind deflection for each bullet at 1000 yards in a 10 mph crosswind. The relative advantage of each bullet will be reduced at closer range, but the order will always be the same. In other words, if a bullet has more wind drift at 1000 yards, it will have more wind drift at all other ranges as well.
Velocity
One very important consideration for bullet selection in a service rifle is velocity. With the shorter barrels and reduced pressures, it’s quite a challenge to get the bullet fast enough that it stays supersonic at 1000 yards. Table 6 shows the remaining velocity at 1000 yards for each of the target bullets considered. As stated above for the Palma rifle, you want your bullets to arrive at the target at a speed that’s faster than the speed of sound which is ~1120 fps depending on temperature. You actually want to stay a little above the exact speed of sound because the negative effects of transonic aerodynamics begin to take their toll as you slow down into the transonic flight region which is faster than 1120 fps. It’s a good rule of thumb to try and stay above 1200 fps in order to avoid the worst of the transonic effects. Table 6 shows that staying above 1200 fps is a challenge for most 30 caliber bullets from service rifles. Bullets that weigh 175 grains or more have the best chance despite their lower muzzle velocities. The velocities in Table 6 are based on standard sea level atmospheric conditions. The retained velocity will be improved if you’re shooting at some altitude above
sea level, and/or the air temperature is greater than 59 degrees. For example, if the air temperature is 80 degrees and you’re shooting at sea level, you can add about 30 to 35 fps to the retained velocity at 1000 yards.
*Special thanks to Joe Sopko who
provided valuable guidance
for the
Service rifle portion of this analysis
Conclusion
There are many things to consider when choosing bullets for long range target shooting. This analysis was meant as a guide regarding the ballistic factors involved in selecting bullets for use with the .308 Winchester in two different classes of rifles used in target competition.
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