SilencerScoutKey info
A suppressor adds backpressure, which drives the bolt carrier harder and raises the cyclic rate.
A faster cyclic rate is not automatically a drawback; an AR-15 has plenty of headroom, especially with a low-backpressure suppressor.
You can slow the cycle with a low-backpressure suppressor, a heavier buffer or stiffer spring, or an adjustable gas block on gas-operated rifles.
We measure single-shot cycle time on a clean, cold bore instead of cyclic rate, because cyclic rate climbs as the chamber and suppressor heat up over a string.
Chapter 1 of 3
What drives Cyclic Rate
Cyclic rate depends on a lot of things working together: the design of the recoil system, your choice of ammo, how clean the firearm is, and more. Doing the full engineering math would mean integrating pressure curves over time, spring constants, kinematic equations and a lot more than this article needs. That gives you a sense of how complicated the topic really is. To keep things simple, we'll use some grade-school physics: F = M × A. We'll also stick to the AR-15, since it's the platform I know best and the one I have graphs for.
Rearranged, the equation to model our cyclic rate becomes A = Force / Mass. Here Force is a measure of the unsuppressed pressure from the gunshot + the backpressure added by the suppressor. You can see from our A = F/M equation if we increase the Force by adding backpressure we increase our acceleration and consequentially our cyclic rate.
What options do we have to reduce this acceleration (cyclic rate)? 1 - We can reduce the backpressure with a low backpressure design. 2 - We can increase the mass with heavier buffers and stiffer springs 3 - delay the bolt opening 4 - And the worst option, we can never clean our firearm so it get's so gummed up with carbon and sludge it slows the bolt speed down.
Chapter 2 of 3
How to reduce Cyclic Rate with Tuning
Every semi-auto firearm has to keep the bolt closed until chamber pressure drops to a safe level. One of the oldest ways to do that is with mass: make the moving parts heavy enough that the chamber pressure needs more time to push them open. This is called direct blowback, and it's still common on pistol-caliber designs and .22 LR. In a blowback gun, adding mass is about the only option you have. A 9mm AR-9, for example, uses a very heavy buffer, roughly 5.5 to 10 oz.
You might look back at F = M × A and ask: "Doesn't adding weight mean the bolt hits harder?" Not really. The gas gives the bolt roughly the same push either way, and spreading that push over more mass means the bolt moves slower. On top of that, a heavier bolt opens later, after chamber pressure has fallen further. Done right, the heavier buffer makes for a slower, softer cycle.
Most centerfire rifles use a locked breech instead, such as the AR-15's rotating bolt, which is unlocked by gas tapped off the barrel. You can still use mass on these to slow the cycle, but you can also control how much gas reaches the bolt carrier. The most common way to do that is an adjustable gas block. Its advantages are that it adds no weight to the recoil system and that some designs can be retuned quickly for different ammo or environmental conditions. Their disadvantage is proportional to the degree to which complex parts and points of failure are added to the weapon.
Pros
An adjustable gas block slows the cycle without adding weight to the recoil system.
Some gas blocks can be retuned quickly for different ammo, suppressors or conditions.
A heavier buffer or stiffer spring is a simple drop-in change that also works on blowback guns.
Cons
Every adjustable part is another possible point of failure.
A gas block turned down too far can short-stroke and fail to feed, especially when run unsuppressed.
Blowback designs have no gas system to tune, so mass is about the only option.
Chapter 3 of 3
What Cyclic Rate Can Tell us About Suppressor Backpressure
You might have noticed our backpressure metric is estimated using the "cycle time" as a proxy. I define cycle time as the interval from the firing pin striking the primer to the bolt returning to battery. The shot is fired with no round left to feed, so the bolt closes on an empty chamber and isn't slowed by stripping a cartridge from the magazine. Trigger reset time isn't included.
So why measure this instead of cyclic rate? Cyclic rate is actually a very dynamic measure that changes throughout the firing of just one magazine. With each shot, the next cartridge seats into an ever hotter chamber, seeing theoretically higher chamber pressures from enhanced combustion as well as more gasses and unburnt powders stacking in the suppressor. A 5 shot sample of cyclic rate should be slower than a 30 shot sample which might be slower than a 100 shot sample. This is not a good way to do science as there are too many changing variables. So instead, I measure 1 cold bore shot on a bolt that was cleaned and re-lubricated. In the image below, you can see the 1ms faster cycle time of shot 5 through the Surefire RC4. The 6th shot is then made even faster as it did not seat a new round when cycling.