February 10, 2014
By Brad Miller, Ph.D.
Compensators are interesting devices. They redirect gas to take the bite out of recoil and reduce muzzle rise. There are two types of compensators for semi-automatic pistols: traditional compensators and bushing compensators.
Traditional compensators attach to the barrel, while bushing compensators replace the barrel bushing and attach to the slide.
Some compensators are called bushing compensators even though they attach to the barrel via threads. They get that name because they include a traditional bushing behind the compensator, but they fall under the definition of a traditional compensator.
Bushing compensators vary by design with respect to important features. Some bushing compensators have chambers and baffles like a traditional design to help trap gas, while others don't.
Opinions vary on how well bushing compensators work, with some suggesting that they don't work at all because the large exit hole allows too much gas out around the bullet. Given the ambiguity of their effectiveness, I set out to compare the recoil of different compensator designs.
The main challenge when testing a bushing compensator is to determine whether it functions like a traditional design. Subjective factors often influence how we perceive recoil; perceived differences are difficult to quantify, so a mechanical method is preferred.
However, even a mechanical device can show innaccuracies when measuring recoil because of the compensator's weight, though this should result simply in a general attenuation in recoil. A more objective criterion is to look for the gas volume effect of compensator function with multiple gunpowders.
Gas Volume Effect
A portion of the recoil force is generated from the weight of the gunpowder itself. For example, if you compare two different gunpowders that produce the same velocity but require different charge weights, the one that requires a heavier charge weight produces more recoil.
Here's why: As gunpowder burns, it produces gas. More gunpowder weight means more gas. Gas volume contributes to the recoil force. More gunpowder weight = more gas volume = more recoil even for the same velocity.
With a non-compensated pistol, gunpowder with a higher charge weight produces more recoil and muzzle rise than a powder with a lower charge weight achieving the same velocity. Since the ports on pistol compensators vent gas upwards, that produces a reciprocal downward force that reduces muzzle rise. More gas volume upward = more downward force.
Therefore, true compensator function will reverse the gas volume effect on recoil. The higher charge weight gunpowder will generate more downward force and produce less muzzle rise than gunpowder with a lower charge weight for the same velocity.
Testing the Gas Volume Effect
Three Para Ordnance pistols were used for the test—a P14-45 in .45 ACP, a P16-40 in .40 S&W and a P18-9 in 9mm Luger.
A Wilson Combat Multi-Comp replaced the factory bushing on all three guns without modification. An Innovative Industries compensator, tested with the P14-45 pistol, also fit without modification. The P14-45 pistol had a separate Les Baer slide, fitted with a Clark .45 ACP threaded barrel using a traditional EGW cone-style compensator.
Based on published loading data, I selected gunpowders that require different charge weights for the same velocity. For the .45 ACP and 9mm loads I used Hodgdon Titegroup, Ramshot Silhouette and Accurate #7 powders. For the .40 S&W loads, I used Winchester 231 and Vihtavuori 3N38 powders.
The loads used 230-grain bullets in .45 ACP, 165-grain bullets in .40 S&W, and 115-grain bullets in the 9mm Luger. I loaded ten rounds of each caliber with two or three separate charge weights for each gunpowder.
Testing with multiple charge weights produces data that permits mathematical evaluation with linear interpolation.
Two parameters were calculated for a specified velocity: recoil (gun movement) and gunpowder charge weight. The specified, caliber-typical velocities I used were 850 feet-per-second (fps) for the .45 ACP, 1,100 fps for .40 S&W and 1180 fps for 9mm Luger.
The guns were fired from a Ransom Rest. Measuring the distance that a gun moves in the Ransom Rest indicates relative recoil. Velocity was recorded with a Shooting Chrony chronograph at approximately 10 feet. Ambient temperature was 70-80 degrees Fahrenheit.
Here is some of the data I collected from my tests:
The big difference between traditional compensators and bushing compensators are the size of the bullet's exit hole. Traditional compensators keep the exit hole as small as possible to trap and redirect as much gas as possible. Their exit hole is just slightly larger than the bullet. Bushing compensators have a large exit hole because the barrel must pass through them with every shot. This allows much more gas to exit forward around the bullet and avoid deflection, making them less efficient than a traditional compensator.
I tested two different designs of bushing compensators — a Wilson Combat Multi-Comp and one from Innovative Industries. The Wilson Combat Multi-Comp has baffle plates and chambers that closely match a traditional compensator made by Evolution Gun Works (EGW).
Both designs have three chambers and vertical ports that are very similar in size, as well as the overall size of the compensators themselves. However, their exit holes are very different in size. The Wilson Combat's exit hole measures 0.627 inches wide, whereas the EGW measured 0.465 inches. The Wilson also has small side ports on the last two chambers that the EGW compensator does not.
The Innovative Industries bushing compensator is a significantly different design. It is essentially a simple tube with a lug to hold it in the slide and a flange to hold the recoil spring plug. It lacks baffles since its thin wall won't accommodate them. However, its exit hole diameter was smaller than the Wilson Multi-Comp's, and measured 0.581 inches — barely larger than the barrel's diameter of 0.580 inches. Its three ports are mere slits in the tube, and small ones at that.
Wilson Combat advertises the Multi-Comp as 'suitable for calibers up to .45 ACP. ' In that light, I tested it with .45 ACP, .40 S&W and 9mm Luger calibers. Clearly, with smaller bullets, there is even greater space for gas to pass forward around the bullet.
Charge Weight Predictions
Charge weight predicted which gunpowders produced the least and most recoil with no compensator. Higher charge weights for the same velocity produced more recoil. This effect is reversed with a traditional compensator.
More Gas = More Recoil
In order to have a basis for data comparison, I tested the .45 ACP data with no compensator and with the EGW compensator. Figure A shows measurements of gun movement with no compensator using the three gunpowders shown in rank order from least to most movement. Titegroup produced the least movement and Accurate #7 produced the most.
This correlates with the weight of the gunpowder required to achieve 850 fps as shown, and is consistent with the principle of more gas = more recoil.
Gun movement with the EGW compensator (Figure B) was just the opposite. Titegroup produced the most movement, Accurate #7 the least. This is a compensator's reversal of the gas volume effect: more gas volume from a larger charge weight of gunpowder for the same velocity = less muzzle rise.
The Wilson Combat Multi-Comp bushing compensator also produces the gas volume effect, though it is not as robust as the EGW compensator. Muzzle rise is significantly reduced with the higher charge weight gunpowders; the net result being that gun movement was roughly equal among the three gunpowders (Figure C). Muzzle rise with the largest charge weight gunpowder, Accurate #7, was the least.
The Innovative Industries compensator's effect on recoil was less obvious. Titegroup and Silhouette powders had about the same recoil, but Accurate #7 still had significantly more recoil (Figure D).
The Gas Volume Effect
The Wilson Multi-Comp also produced the gas volume effect with .40 S&W and 9mm Luger. With the .40 S&W, the larger charge weight 3N38 gunpowder produced more recoil than the 231 powder with no compensator and less recoil with the Multi-Comp.
The 9mm Luger results are similar. Charge weight is clearly associated with more recoil with no compensator. With the Wilson Multi-Comp, the results look similar to its .45 ACP data. Recoil levels are very similar among the three gunpowders, with Accurate #7 having the least recoil.
Comparing the percent of recoil reduction with the compensators shows how effective they are compared to having no compensator, and also illustrates whether or not the compensator shows the gas volume effect.
The EGW compensator reduced recoil by an average of 46-percent with the .45 ACP and showed clear evidence of the gas volume effect: gunpowders with larger charge weights had a greater reduction in recoil.
The Wilson Combat Multi-Comp reduced recoil by an average of 30-percent in the .45 ACP and showed the gas volume effect for all three calibers.
The Innovative Industries compensator produced an average of 14-percent less recoil. A gas volume effect was not obvious, however, with the Accurate #7 powder showing no greater recoil reduction than the Titegroup powder.
The Wilson Combat Multi-Comp unexpectedly produced a higher percent of recoil reduction with the 9mm Luger cartridge than the larger calibers. This seems counterintuitive, because the smaller 9mm bullet would allow even more gas to escape forward around the bullet and avoid deflection by the baffles.
It's likely related to the lower recoil of this cartridge in general, and that the 9mm still required an average of 84-percent of the gunpowder charge weight of the .45 ACP, but with bullets half the weight. Similar gas volume with much lighter bullets might make compensator function more efficient.
Comparing other data I have from testing a traditionally-compensated .38 Super supports this idea. Its charge weights with the same gunpowders and 115-grain bullets are nearly the same as the .45 ACP with 230-grain bullets. It shows an average 74-percent reduction in recoil compared to the .45 ACP's average 46-percent reduction in recoil. Thus, lots of gas and lighter bullets means greater recoil reduction.
The Wilson Combat Multi-Comp bushing compensator functions like a traditional compensator and for a surprisingly broad range of calibers, though it's not as efficient as a conventional compensator, because it produces less recoil reduction. The Wilson's large exit hole traps less gas for use in the ports, but it still redirects enough gas upward to help tame recoil and muzzle rise by an average of 30-percent in the .45 ACP.
The Innovative Industries compensator had a lesser effect but does show a slight reduction in recoil. Its small ports vent far less gas than the large ports of the other compensators. But the fact that it shows some reduction in gun movement makes an important point about how compensators work. Compensators reduce recoil and muzzle flip by redirecting gas. Any redirected gas creates force. The more gas they deflect, the more force they apply to counter recoil.
If you're looking to buy a bushing compensator, check the details of their design. The ones with chambers and baffles are more effective than those without them. Baffle plates trap and redirect more gas, and compensators are all about gas.