September 22, 2014
Many organized shooting sports require ammunition to achieve a minimum power level to qualify and be scored in a specific division. This is known as power factor.
Power factor is calculated by the formula: bullet weight times velocity divided by 1000. Power factor requirements are determined by which division one competes in, and might be caliber specific.
A common distinction is between "Minor" and "Major" power factor, and it determines the score value of peripheral hits. For example, in some divisions of USPSA competition, in order for pistol ammunition to qualify for Major, it must be at least .40 caliber and achieve a 165 power factor. Minor power factor ammunition must achieve a value of at least 125.
Many of these sports are timed events, and a light recoiling load can allow faster follow-up shots, giving the shooter a speed advantage. As we know, every fraction of a second can determine who wins the match.
This begs the question from many shooters: When reloading for competition, which bullet weight gives you the edge?
The following test will determine the optimal bullet weight(s) with the lowest possible recoil while still achieving a desired power factor.
Power factor requirements theoretically put all ammunition on even ground with respect to recoil. If you calculate the recoil force, you'll find that all bullet weights produce the same recoil when loaded to the same power factor. For example, using common bullet weights in the .45 ACP with the goal of a 165 power factor: a 230-grain bullet at 718 feet-per-second (fps) has the same recoil as a 200-grain bullet at 825 fps or a 185-grain bullet at 892 fps (Table 1). Hence, there is no obvious advantage of one bullet weight over another.
However, this assumes that the gunpowder charge weight is the same for all the different bullet weights. You see, it matters how much gunpowder is required to achieve the necessary velocity, because gunpowder weight affects recoil force.
Recoil for different bullet weights in the .45 ACP was tested using Winchester 231 gunpowder. The following copper-plated bullets were tested: Berry's 230-grain RN, Rainier 200-grain RN and Berry's 185-grain HBRN (Figure 1). All were loaded to the same 1.240-inch cartridge overall length (COL).
To test which bullet weight gives you the edge, three or four different charge weights were prepared with each bullet weight. Ten rounds were fired with each load. Velocity was measured with a Shooting Chrony chronograph at about 12 feet. The test gun was a Para Ordnance P14-45 and was fired while held in a Ransom Rest (Figure 2).
Recoil was measured two ways. The first method measured how far the gun moved in a Ransom Rest. The gun is secured in the device's rocker arm, and this assembly pivots upward when the gun fires, much like a person's hands and arms move when firing a handgun.
The second method of measuring recoil was mathematical and used the bullet weight, gunpowder charge weight and velocity. The use of multiple charge weights allows you to calculate the gunpowder weight required to achieve the 165 power factor with linear regression. This value was then used to calculate recoil force. Linear regression was also used to calculate the distance the gun moved for 165 power factor.
Do these different bullet weights achieve the same power factor when loaded with the same gunpowder charge weight? No. Not even close. The same amount of gunpowder produced similar velocity regardless of bullet weight (Table 2), though the heavier bullet usually went faster with the same charge weight.
Lighter bullets need more velocity to reach the 165 power factor, and that requires more gunpowder (Table 3). The 200-grain bullets require 15-percent more gunpowder, and the 185-grain bullets require 27-percent more gunpowder than the 230-grain bullets.
Do the bullets produce the same amount of recoil at the same power factor? No. The 185-grain bullet produced the most recoil, and the 230-grain bullet produced the least. This was confirmed by both methods of measuring recoil. The Ransom Rest results (Figure 3) show that the 200-grain bullet had 5.9-percent less movement, and the 230-grain bullet had 9.1-percent less movement compared to the 185-grain bullet.
The mathematical results showed that the 200-grain bullet had 2.2-percent less recoil, and the 230-grain bullet had 5.6-percent less recoil force compared to the 185-grain bullet (Figure 4).
While power factor attempts to equalize the recoil playing field, heavier bullets have a slight edge. They have less recoil because they require less gunpowder.
How does it feel?
Many shooters claim that the recoil of light and heavy bullets feels different when loaded to the same power factor. Light bullets tend to have a quicker "snappy" feel while the heavy bullets tend to have a slower "pushy" feel. Shooters differ on which they prefer.
Light bullets must go faster than heavy bullets to achieve the same power factor, so they accelerate faster and spend less time in the barrel. This means a faster recoil impulse. The light bullets' shorter barrel time and additional recoil likely combine to make their recoil feel different from heavy bullets.
One possible confound to comparing bullets of different weights is that they might differ in their construction (lead hardness, jacket hardness and thickness) which could affect their friction coefficient, which could affect their velocity which would affect the calculations. I tried to control for this by using the same type of bullet (plated), albeit two different brands.
COL is another variable that can affect these results since it affects pressure, which affects velocity. I tested bullets of the same nose shape loaded to the same COL so that the internal volume is proportionately affected by the difference in the length of the bullet itself. The two Berry's bullets have virtually identical nose shapes, so their difference in internal volume is ideal for this test when they are loaded to the same COL. The Rainier bullet nose is slightly pointy, which makes it a little longer than it would be if it had the same nose shape as the Berry's bullets. However, this slight deviation in shape that would result in a small deviation in internal volume did not adversely affect the main results. Even if the Rainier bullet data was excluded, the two Berry's bullets (230- and 185-grain) illustrate the point very well.
This comparison has been run several times with different calibers, bullet brands and gunpowder (.40 S&W with mixed brand 165-, 180- and 200-grain jacketed bullets; .38 Super with mixed brand 115-, 130-, 147- and 180-grain jacketed bullets), and the result is the same; heavy bullets produce less recoil and require less gunpowder than light bullets for the same power factor. As long as you're not comparing bullets of drastic differences in design and construction, such as jacketed versus frangible, or working with vastly different COLs, this rule holds.
Different bullet weights produce different amounts of recoil when loaded to the same power factor. Light bullets produce more recoil than heavy bullets because they require more gunpowder to achieve their higher required velocity. The snappier recoil that people experience with lighter bullets likely results from their faster acceleration.
The formula for ammo with the least recoil is to use heavy bullets in combination with a gunpowder than produces the least recoil. Which gunpowder does that? Select a gunpowder that requires the smallest charge weight to achieve your desired velocity, because less gunpowder weight for the same velocity means less recoil. Check your reloading manual for that information.
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