More air, less filling - some thoughts about the PCP performance/power curve

by Harley Ayre

Your precharged airgun is a lot like a musical instrument. To a beginner, all musical instruments look and sound pretty much the same, just as similar models of precharged airguns look and feel similar to beginning shooters. After you spend some time with them, however, each gun begins to let you in on its secrets, one of which is that it might not operate exactly like you think it should! Let's consider some of the performance factors that aren't included in the manual!

Starting with max fill pressure
In the last post I told you that small pressure gauges are not always accurate - in fact they are rarely exactly on the money. So, it behooves you to learn the pressure your gun likes best according to the gauge you use. What if the max fill pressure turns out to be 300 psi below the recommended maximum in the manual? Use that figure as your stopping point, not the number in the book. You'll still get the same number of powerful shots as if you had filled the gun to the printed maximum WITH ONE DIFFERENCE: Your first shot will be right on the power curve instead of being weak and wimpy.

Okay, so that's a repeat of the last post, but here's a twist. It can also work the other way! Your gauge may read high and you may have to establish what the top filling pressure really is. Don't guess about this! Use a chronograph to determine the top and bottom pressures of your airgun's performance/power curve. Guessing gets you nowhere fast. If you don't have a chronograph, use the numbers given in the manual.

What do you mean, the BOTTOM of the performance/power curve?
We learned how to establish the TOP of the performance/power curve in the last post. Where the bottom of the curve is located is up to you. It's that point where the accuracy of your gun starts dropping below your standards of acceptability. That will be at a different pressure level for a guy who shoots bullseye targets at 50 yards and another guy who only hunts rats at the dump. But at some point, both shooters will notice they aren't getting the accuracy they want from their guns. If they then connect their fill device (scuba or pump) to the gun and start to fill, the point at which the gun starts accepting air defines the bottom of the performance/power curve. On some guns, you have to be very diligent to notice when the needle on the gauge slows down just a bit. Falcons are often very smooth at this point, while most Daystate rifles respond with an audible click and a big jump of the needle. You have to watch and learn each gun.

Is there only one power curve?
An airgun usually has more than one flat spot in its power curve, where velocity doesn't vary by much and accuracy will be optimum. The reason we speak of only one curve is because we are generally seeking the most power the gun can give with the best accuracy. Shooters who take the time to experiment with a chronograph can find all sorts of additional places the gun likes to shoot on the performance/power curve. After hearing about this from a Talon SS owner, I discovered three different places where my SS liked to operate. True, I didn't get the full power the gun had to give on the top curve, but I did get some spots that were useful for indoor target shooting. And, let me tell you, pumping a gun to 1,500 psi is a heck of a lot easier than going to 3,000!

A case where less was more
I knew a man who owned a big Korean Career 707 air rifle. Straight from the factory, those .22 caliber guns topped 50 and even 60 foot-pounds for the first couple of shots! But this guy didn't want that. He didn't want 15 to 20 shots from his Career, each of which would be at a lower velocity than the shot before, until shot 20 was shooting 100 f.p.s. slower than shot number one. He wanted a useful band of 20 to 25 shots that didn't vary by more than 30 f.p.s. so he could hunt squirrels and have some chance of hitting them. He got it, too, but not where most Career 707 owners would expect it to be. When he filled his rifle to 2,000 psi and shot until the gun had only 900 psi remaining, he got a nice group of 30 shots with .22 caliber Crosman Premiers in the 600 to 650 f.p.s. region. The Career will launch a .22 Premier at over 1,100 f.p.s. when the gun is filled all the way, but this guy was filling only to the point where most shooters stop shooting! He had good success dropping squirrels from the trees at pellet velocities most airgunners would laugh at. But, heck, we know the British airgun hunters have to shoot their .22 Premiers at 600 f.p.s. and less to stay within their law, and they do a lot more airgun hunting than we do.

Put that chrono to work. Even better, you don't need a chrono!
This time spent with your air rifle justifies the expense of that chrongraph a lot more than using it as a radar gun to see how fast you can shoot. And, don't despair if you don't own a chrono. There is a chrono-free method of doing this that just as good. Simply shoot at a distant target and see what kind of groups you can get on lower fill pressures. I like to place my target out at least 40 yards, because that really shows the accuracy potential. The aimpoint will not be in the same place that it is on a full fill, but that's a simple matter of a scope adjustment. Know what else? Your gun will be quieter and better behaved, too! All you Condor owners who brag that your rifle will shoot through a two-by-four can now enjoy 40 shots or more at reduced velocities. Yes, I am aware that you can do the same thing with your power adjustment wheel (don't those AirForce gun owners make you jealous?), but I'm talking about shooting with lower air pressure. If you are a pump user, I bet I have your attention now!

The top end of performance isn't the only place your PCP gun shines. With a little exploration, you might find that you own several wonderful airguns in one! While you're finding out, you'll be shooting! Nothing better than that!