Precharged pneumatics, regulators and power adjusters: Part 3
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Power adjustment
- Striker (hammer) blow
- Transfer port control
- Michael — this is for you
- Finally — how practical is adjusting power?
Today I will try to finish my answer to Michael about power adjusters on precharged pneumatics (PCP), which I have broadened to include air regulators. Please read Parts 1 and 2 to see what has gone before.
Michael, there are several things an owner can do to adjust the power of his PCP. A longer barrel will often increase power, but that’s not immediately within the owner’s grasp. We are talking about adjusting power of a specific gun by means of things that are easy to do, without changing or modifying the rifle.
Striker (hammer) blow
Power can be adjusted easily by controlling the striker blow to the firing valve. A harder blow will force the valve open wider or for a slightly longer time. Either one of those or both of them means that more compressed air can flow out of the reservoir to power the pellet. As long as the pellet is inside the barrel, the more compressed air that gets behind it to push it, the faster it will go.
The weight of the striker is fixed. A heavier striker could apply more force to the valve stem, but I have not seen a system where the user was able to change the striker weight on the fly. That leaves either adjusting the striker spring tension, or the length of the striker’s travel as the only two ways of adjusting power. The simplest method of adjustment is by increasing and decreasing the tension on the striker spring. This is done with a threaded screw of some kind that presses on the end of the spring opposite the end that’s in contact with the striker.
In the graphic above the spring at the top is relaxed for a less powerful strike. When the spring is compressed, the striker hits the valve harder.
If you can adjust the length of the striker’s stroke, you increase its inertia. That increases the force with which it hits the firing valve.
That should be very clear and obvious. But bear in mind that if there isn’t much compressed air in the reservoir, you can hit that valve as hard as you want and nothing more will happen. Or, if the valve only opens so far, you can hit it hard enough to destroy it and still get no more power. The striker can only control power so far before it runs out of capability.
Transfer port control
A better way for the user to control power in a PCP is to control the size of the air transfer port. That’s the same as controlling the gate at a rock concert. There may be thousands of people waiting to get in, but the size and efficiency of the gate controls how fast the auditorium fills. Control the size of the air transfer port and you control how much air can flow through in the time the pellet remains inside the barrel — which is the only time that matters.
The transfer port on top is wide open and will pass as much air as possible in the time the valve is open. The port on the bottom has been restricted and can only pass a fraction of the air.
The restriction can be as simple as a screw that intrudes into the transfer port. Anything that hinders the flow of air will work. Some guns have definite power settings, while others have an infinite adjustment between the two limits.
Michael — this is for you
The Benjamin Marauder, has all of these adjustments. That’s quite uncommon. I agree that the way the Marauder adjustments work — via hex screws buried inside the receiver — neither adjustment is as easy to make as those found on some other guns. But the Marauder gives you adjustment flexibility that surpasses most other PCPs. However, that higher flexibility brings in some complexity.
As you adjust the Marauder, since you are adjusting the power of the striker via its spring and also the length of the stroke, plus controlling the air flow through the transfer port, you are changing the optimum air pressure range at which the rifle operates. Simply stated — you’re changing the fill pressure of the rifle. It would be wonderful if this could be done with a single external adjustment knob, but it can’t. The power of the striker and the efficiency of the air flow through the transfer port are interdependent. Each one affects the other.
Think of this like an audio amplifier that also has equalization. You can adjust the highs and lows, but if you change the volume, the highs and lows need to change, too.
The last subject I will cover is stability through the range of adjustments. Some PCPs are very stable (small velocity variances) when they are in a certain range of their power adjustments. The TalonSS, and in fact all AirForce sporting rifles, like the medium to high part of their power range. Velocity may vary by less than 20 f.p.s. in that part of the power range, but if the power goes to the lowest level the variance may open to 40 to 60 f.p.s. You’ll get more shots, but they will also vary more. However, install a MicroMeter tank and that turns around. That’s because the AirForce tanks are where the firing valves live, and they are tuned for what is expected from that tank.
Finally — how practical is adjusting power?
This is the advice that GunFun1 gave Michael about adjusting airgun power. I would have said something similar.
Michael, Think about this also with the power adjustment. Yep it’s easy to dial the power down for the pest bird in the yard and up more for the squirrel out at 60 yards.
But here’s what to think about. What happens to POI/point of impact when the power is changed? It goes up or down. So that means now you have a gun whose shifting impacts at different power settings need to be learned.
I use to do that with my first Talon SS and Marauder. But I found out after time that it’s best to tune the gun for what velocity and fill pressure you want and leave it alone. In other words don’t keep changing the gun settings for a particular time of shooting.You get a much more consistent gun that way by leaving it set at that particular setting. And, what I mean by that is a accurate gun and a accurate person shooting that gun.
When you’re pesting or hunting you need to know where that pellets going to hit in relation to where you aim. Sometimes it’s hard enough learning the holdover or under at one setting, let alone multiple power settings.
I will just add this. It used to be popular to have a rifle and pistol that were chambered for the same cartridge. People said if you ran out of rifle ammo, just use your pistol ammo. Sounds nice and neat, doesn’t it? Well — it isn’t! It’s messy. A rifle handles a cartridge completely different than a handgun, and instead of using the same round in both what you end up doing is compromising. Your ammo doesn’t work as well as it could in either the rifle or the pistol.
In this series we have talked about pressure regulators and what they can and cannot do. Then we talked about balancing a valve to operate within a certain pressure range. Finally we talked about power adjustments — what is done to make them work and how they affect the performance of the rifle.
If you remember nothing else, take this away. A regulator works best when it is set at the pressure the airgun works best at. Anything less and the reg. is just along for the ride.