November 05, 2014
By Brad Miller, Ph.D.
Previous tests show that light bullets have more recoil and muzzle rise than heavy bullets when driven at the same power factor, because light bullets need more gunpowder for their required higher speed. The greater amount of gunpowder produces more gas that adds to the recoil force.
What happens when you put a compensator on the barrel? A compensator uses gas to reduce muzzle climb. Tests show that the extra gas provided by a gunpowder that uses a larger charge weight reduces muzzle rise more, even though they produce the same velocity with the same bullet . The additional gas provides more force to counteract the muzzle rise more effectively.
This suggests that a compensator might reverse the effect of lighter bullets on recoil. Because lighter bullets require more gunpowder, this provides more gas to counteract muzzle climb, and they might have less muzzle rise than heavy bullets when loaded to the same power factor.
This was tested in two calibers, the .38 Super and .45 ACP. The .38 Super is commonly used in competition pistols where compensators are allowed. The .45 ACP is less common in a compensated competition pistol because the larger diameter round reduces magazine capacity, but it was included here to make a direct comparison with data from the previous article on bullet weight and recoil. Both guns have three-port compensators with ports only on the top (Figure 1).
The .38 Super ammunition compared 115-, 124- and 147-grain Hornady XTP JHP bullets (Figure 2) loaded to a cartridge overall length (COL) of 1.240" in Starline 38TJ brass (a rimless version of the 38 Super) with three different charge weights of Vihtavuori 3N38 and Federal 205 (small rifle) primers.
The .45 ACP ammunition compared 185- and 230-grain RN Berry's plated bullets loaded to a COL of 1.240" in Sellier and Bellot brass with three different charge weights of Winchester 231 and CCI 300 primers.
The gun was fired from a Ransom Rest. Measuring the distance that the gun moves in the Ransom Rest indicates relative recoil. The rocker arm that holds the gun pivots upward when the gun is fired. Since the compensator ports vent the gas upward, the gas produces a reciprocal downward force that reduces muzzle rise by reducing how far the gun pivots. Thus the Ransom Rest provides an objective method to measure gas effectiveness with a compensator.
Linear regression was used to calculate gun movement and gunpowder charge weight at a power factor of 165, the power factor for Major scoring in USPSA practical shooting.
As expected, light bullets required more gunpowder to achieve their required velocity. In the .38 Super, the 115-grain bullets required 9.5 grains of Vihtavuori 3N38, the 124s required 8.7 grains and the 147s required 7.2 grains. The 185 grain .45 bullets required 6.3 grains of Winchester 231, while the 230 grain bullets required 4.6 grains.
The result of lighter bullets requiring more gunpowder means that they produce more recoil force. The calculated recoil force for both calibers is shown in Table 1. In the .38 Super, the 115-grain bullets had 3% more recoil force than the 124-grain bullets, which in turn had 6% more recoil force than the 147-grain bullets. In the .45 ACP, the 185-grain bullets produced 7% more recoil force than the 230-grain bullets.
Even though light bullets produce more recoil force than heavy bullets at the same power factor, the compensator uses the additional gas provided by the extra gunpowder to reduce their muzzle climb more than heavy bullets. In the .38 Super, the 115- and 124-grain bullets had 21% and 13% less muzzle rise, respectively, than the 147-grain bullets (Figure 3). The 115-grain bullets had 9% less muzzle rise than the 124-grain bullets.
The .45 ACP demonstrated the same greater reduction in muzzle rise with the lighter bullet. The 185-grain bullets had 6% less muzzle rise than the 230 grain bullets.
The .45 ACP test in "Power Factor & Recoil: Which Bullet Weight Gives You the Edge?" used the same Berry's bullets and Winchester 231 gunpowder, which allows a direct comparison of how much muzzle rise was reduced with a compensator. With no compensator, the 185-grain bullets had 9% more muzzle rise than the 230-grain bullets, compared to the 6% less muzzle rise with a compensator in this experiment.
Figure 4 shows data from the previous article compared to data from this experiment. Note the reversal of muzzle rise with the light and heavy bullets with the use of a compensator. Crunching the numbers shows that the compensator produced a 30% reduction in muzzle rise with 230-grain bullets and a 40% reduction with 185-grain bullets.
More gas volume should translate into higher gas pressure at the muzzle when the bullet exits the barrel. This can happen even if the larger charge weight does not produce a higher peak chamber pressure, as demonstrated when comparing two gunpowders with different burning rates.
Higher gas pressure at the muzzle should indicate greater upward force deflected by the compensator's baffle plates and ports. This would produce more downward force to reduce muzzle rise. QuickLOAD software (version 188.8.131.52) was used to estimate gas pressure at the muzzle to see if it corresponded with the greater reduction in muzzle rise with the light bullets. It did.
Calculations with the .38 Super data showed that the light bullets had higher muzzle pressure than the heavy bullets. The 115-grain load had 10% more muzzle pressure than the 124-grain load, and 34% more than the 147-grain load (Table 2). QuickLOAD does not have the Berry's .45 caliber bullets in its database, so I substituting 185- and 230-grain Hornady HAP bullets for the calculations.
Note: QuickLOAD's calculated pressures are estimates. The actual pressure can be different, but the trend of pressure differences (more, less) between bullet weights should be the same. The calculations showed that the lighter 185-grain bullet had 45% higher muzzle pressure than the 230-grain bullet (Table 2). Thus, higher muzzle pressure can overcome what would normally result in more muzzle rise with no compensator to produce less muzzle rise with a compensator.
A compensator changes how a gun responds to light and heavy bullets when loaded to the same power factor. Light bullets produce less muzzle rise because they use more gunpowder, and that means more gas to enhance the compensator's effect. This is just the opposite of what happens when you don't have a compensator where light bullets have more recoil than heavy bullets. If you're loading ammunition for a compensated pistol, light bullets combined with a gunpowder that provides a lot of gas (large charge weight) will produce the least muzzle rise.
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