I was shooting the Prairie III out the back door the other day and noticed something. With the gun set to its lowest power setting it started getting louder after the 4th or 5th shot. I was shooting with the gun attached to a regulator but I had forgot to turn on the pressure. As the gun 's internal pressure dropped it was starting to shoot harder. This gun has a very effective shroud so it was easy to tell when the gun started shooting at the top of the power curve. The odd thing is that I had never figured out that at low power the gun's curve is well offset from where it is on the higher settings.

This gun is designed to shoot long range off a bench at around 250 FPE. I like to do that when I can but I also love to shoot this gun at its lowest power setting when I'm home. Even with the long shroud; at high power, the gun makes more noise than you want bouncing off the walls inside the house. When I set it to low, I can shoot with a very quiet gun that is still making 80+ FPE. I've found that even though I can slow the gun to under 700 FPS, I can get fantastic groups at 50 yards and below at this low setting. That's why the gun is set up on the shooting bench most of the time inside the back door 43 yards from the target wall (yes, there is no woman living here!). Anytime I want, I can go shoot a group or anything that might be placed in front of the wall and relax a bit.

Anyhow, when I realized that the gun was getting louder and then figured out why, I just turned on the regulator to about 3200 psi and got the gun back down to a nice low "click"/"poof" level. Even when it was at 2800 PSI it was quiet by any standard (especially considering that the power was up around .22LR level) but with the pressure dialed up a bit, the sound and feel and groups were just about perfect.

Two of the groups I shot during that session.

Tat's a 116gr .308 slug stuck in that group. Yeah, 5 shots would have made a bigger group but I'm always fiddling around and three shots tells me what I need to know. I've already figured out the gun is accurate, getting three shots this tight is just cool...

Great groups (I'm jealous.... )!

But Ive read this post a few times... still dont understand how adding pressure can decrease power. you had it connected to an external airsource, but had not turn it on..... and the gun was running on the internal source..... what makes it run harder? (how might be the better question)...

Dan

Hi Dan,

At first I didn't realize that I didn't have the external source turned on. That's when I noticed that the gun was starting to shoot harder.

What I found was that with the external air turned on and set to 3200 psi, I was shooting "above" the curve. Every unregulated PCP has a "curve" of air pressures that produce at least a few consistent shots at a given velocity/power level. If the air pressure is to low, the gun shoots softer and if the pressure is to high the gun shoots softer until the pressure drops into the curve. With higher air pressure it takes more work to open the valve against the pressure. If you filled the gun to a high enough pressure, it might not shoot at all because the hammer and spring would not be strong enough to release any air. I just sort of stumbled into the fact that I was shooting on the high side of the pressure curve.

I've since kept the pressure on the high side for shooting because I am not maximizing power when shooting out the back door and the high pressure gives me a nice quiet and crisp shot. I could get the same lower power by shooting on the low side of the curve but then I might be close to hammer bounce....

An additional factor which is relative: the gun has a hammer that's not only adjustable for spring pressure, but adjustable for hammer stroke too. There's a number of ways Jerry can set up the gun. What he's doing here is just a quick and dirty way of adjusting the tune for the purpose of easy/quiet shooting.

Gary

So the higher pressure makes the mechanism work harder, and therefore less efficient....?

Ok that makes sense.

Yes - there's a "balance" between the back pressure holding the valve closed, and the mechanism designed to open the valve briefly. With more "pre-load" on the mainspring, and a greater hammer travel, Jerry's gun has plenty of "omph" to open the valve at higher pressures. However; for the purpose of shooting shot and quiet, Jerry has chosen to throw the balance somewhat off. By having the spring lighter and the hammer travel shorter, there will be a new balance (new psi peak) where the gun will give a nice metered string of slots. That new peak psi will be lower than the previous higher power setting.

Understanding that, Jerry has choosen to de-tune the gun even farther by simply going past the new balance point, to a pressure where the mechanism is at a slight disadvantage. It brings his gun down into the 80 fpe range where he can (with his huge backstop) shoot on his property. Then, nobody hears the gun ... they just wonder what in the heck that "thwack - thwack" sound is of the slug smacking the target wall!

Gary

Jerry, this is a wee bit off topic but relevant to external air supply. I have never tethered any of my big bores to a scuba bottle but intend to do so soon. Dumb question, but do you refill after each 1 or 2 shots....I assume you can't keep the air supply on constantly especially when you are using 4500 psi externally?

Hi Neil,

When I shoot with a tether the tether is connected to a regulator that is screwed into the carbon tank's valve. I got mine from AirHog and it has done well. The gun only sees the pressure I select on the regulator and then self fills after every shot or when the pressure from the regulator exceeds the gun's internal presuure plus valve spring resistance (if any). With the Woodsman, I can hear it "take a sip" after every shot. The Prairie III's intake valve is silent so I don't know if it fills after every shot or every couple of shots.


This picture doesn't show the regulator all that well but you can see the valve body, two gauges (tank pressure and output pressure) and the control knob.

Ditto Neil:

I've got the same setup from Airhog for my big bores:



I had Van give me a five foot microline whip to make tank placement easier. I can even muzzle-load my Alpine 45 without de-gassing, or sit the tank on the ground and feed my Scout 45 roundball all day!

As you can see in this picture, the female Foster connector on my rig also has a 90 degree elbow to help get the hose out of your way. Again, courtesy of Van at Airhog . . . he'll hook you up.

-- Jim

Thanks Jerry and Jim, I get the picture now, I see I have some more technology to purchase.....I hope Airhog will export
Jerry, love your dining room setup, I think you said there was no woman in the house. Yep, I can believe that
Jim, I think I could adapt to that comfort you have there

Neil,
I'm looking for an independently wealthy home maker that won't find having a rifle set up on a bench facing out the back door at all times.