Tuning Efficient regulated tuning for beginners

[Booder98]

https://hardairmagazine.com/ham-columns/tuning-regulated-pcp-airguns/

From what I can tell, this article is saying that if you turn the hammer spring in (in means increased spring compression) you get to a state where air is being spent after the pellet has already left the barrel. The 'plateau' in his terminology. But that turning the spring out too far can lead to a higher standard deviation in shot speeds. That the hammer tapped the valve on its sweet spot and stayed open longer, you held your mouth wrong and the valve closed early, etc. For the author, the most efficient setting to give you the lowest standard deviation is to turn the hammer spring down until your shot speeds are around 3% lower than the maximum possible shot speed. So if the plateau is 1000 fps (round number), that you turn the hammer spring in until you get no increase in speed, then you turn the hammer spring out until you get to 970 fps.

What I'm asking is, "Did I get this right?" Because it seems like a fairly simple recipe to follow. We get the benefit of the author's research, and crank the spring in until the speed doesn't change, then crank it out until it drops a little bit. Viola!

 

[CyberDyneSystems]

I'm no expert, but it sure sounds right!

 

[Revoman]

"What I'm asking is, "Did I get this right?" Because it seems like a fairly simple recipe to follow. We get the benefit of the author's research, and crank the spring in until the speed doesn't change, then crank it out until it drops a little bit. Viola!"

I believe it should read "until the speed drops off", then back it off 3-5% from that point.

mike

 

[nervoustrig]

You got it! That’s all there is to it.

Just one point of clarification...this state of tune is reasonably efficient, it does not maximize efficiency. Efficiency is higher when you back off the hammer spring tension even more but it comes at the expense of consistency, meaning the extreme spread will suffer. Back off far enough and you’ll start seeing the velocity swing wildly up and down from shot to shot. The gun is operating at a state of partial valve lock...the hammer is barely hitting the valve hard enough to knock it open.

Adjusting to the knee (95 - 97% of maximum velocity) is akin to an unregulated PCP operating at the sweet spot on the bell curve, meaning slight changes in operating pressure (regulators aren’t perfect) do not affect the velocity in a meaningful way.

 

[TDK]

The 95-97% window has shown best accuracy and shot to shot consistency in my Impact, supporting this theory. 

To try and save tuning time, what I did last round was to set my M3 Macro-16, Micro-5 (full hammer power) and blast a few noisy rounds down range,watching the velocity. Adjust regulator until the velocity max is where I want it (about 920). Then back the settings down to achieve the target 95-97% window (about 880). A lot faster than shooting and then plotting full strings. If you wanted to fine tune, then dial down to 95% and shoot up to 97%.

I think in one of his competition guns, Ted said was running really low reg pressure in his .30, like 80-90bar. His velocity was around 850 if I remember right. This makes sense if he is setting his velocity to this 95-97% point for best consistency.

In my mind, if you wanted to squeeze all the potential accuracy out of a regulated gun, you'd set regulator, shoot a few groups between 95 and 97% of max, then slightly bump reg (which would bump the max and also the 95-97% point). Rinse and repeat. This would help you find the intersection of best harmonics, projectile's fav speed, and shot to shot velocity consistency. But thankfully JSB pellets seem to shoot great between 850 and 900 because I don't have the time or pellets to do testing that extensive!

 

[JungleShooter]

I have run a PP700 at a MV 10% below the plateau, and for my purposes the variation of the muzzle velocity between shots wasn't terrible at all (of course, every gun is different, measure yours).



Additionally, when tuning the gun lower below the plateau the gun gets quieter — if that is important to you.



Matthias

 

[nervoustrig]

Agreed. For short range plinking or pest control, a regulated PCP dialed way down is a beautiful thing.

A while back my buddy invited me over to cull an excess of gray squirrels from his yard. He lives in the suburbs with neighbors on 3 sides. I needed to be discreet. The longest shot would be only 30 yards so the night before I took a regulated .22 making about 30fpe with JSB 18.1gr (860fps) and simply backed off the hammer spring tension a few turns and test fired a much lighter 13.4gr.

It sounded like the pellet barely dribbled out of the barrel. Then I looked down and the chronograph said 720fps. I thought it was a bad reading. It wasn't. It was running about half power and the super short dwell made it sound like a twig snapping under foot. The velocity was jumping around some but all the pellets were landing in a dime at 25 yards. Just right for the task at hand.

The next morning the first squirrel fell to a brain shot at 22 yards. His buddy 4 feet away just looked quizzically at him flopping around on the ground and went back to foraging. I handed the rifle to my son and he took the second one just a few seconds later. That morning the new rifle went 6 for 6 between 20 - 30 yards and drew no unwanted attention from the neighbors:



So if you have stretches where you're shooting at modest distances, give it a try. Quiet, efficient, and a very pleasant shot cycle.

Shooting in the basement in the winter? Oh yeah.

Shooting out past 50 yards? Keep it tuned to the knee.

 

[Vetmx]

I have run a PP700 at a MV 10% below the plateau, and for my purposes the variation of the muzzle velocity between shots wasn't terrible at all (of course, every gun is different, measure yours).



Additionally, when tuning the gun lower below the plateau the gun gets quieter — if that is important to you.



Matthias

It’s because you have a 10 inch barrel that 10% is working better than 3%. You don’t have enough runway to make use of only a 3% reduction from the plateau. If you want to try 3% below the plateau, you’re going to have to turn in the valve return spring but you will wind up at the same velocity when all is said and done. It’s just the life of a short barrel.

 

[Booder98]

Bumping the hammer down to 97% of max did seem to help with efficiency - I'm using less air per shot for faster shots (turned up the reg a little bit before all this) now. I'm getting close to 100 shots at 1600 psi on the reg.

I couldn't reduce the hammer spring lower because I was already at zero turns. I also ran into the "can't turn past 3 turns" issue. I had to get a T-handled allen wrench, make a face, cross my fingers, and crank away. I've seen on the web where a guy put a lighter spring in his Avenger to give him a wider range of settings with the hammer spring. I am willing to bet in my case that I'm running low regulator settings; I'm setting it up at the moment for 30 yd plinking with CPHP.

 

[nervoustrig]

A short barrel is actually somewhat more sensitive to being tuned well below the knee. Ideally we want the dwell to be identical for each shot. For a rifle-length barrel, that's generally somewhere in the range of 1 to 2ms. A short barrel needs less dwell, more like 0.5 to 1ms. So let's say we back off the hammer spring tension and introduce a 0.1ms variation in the dwell. This variation represents a 5-10% deviation from the ideal for our long barrel, but a 10-20% deviation for our short barrel.

Granted, we can often tolerate a greater variance from a pistol or carbine because we use them at short distances where the extreme spread isn't as important.