Tuning Tuning for efficiency and shot count (without forgetting precision)

How to tune for efficiency? And how to tune for shot count? And what impact does it have on the accuracy?

I've noticed that this is being discussed in multiple topics all the time over multiple forums and platforms and thought it would be a good idea to elaborate a bit on this. Feel free to add your experiences and questions, many knowledgeable tuners around here that can help and many that want to learn :)

So, let's clear something up first: efficiency in terms of liters of air per shot and shot count unfortunately do not go hand in hand. Shot count is the optimization between efficiency and headroom in pressure (between reg and fill pressure) which isn't the most efficient tune. I've written something about it in a topic once which was aimed at a short barreled FX Impact, short barrels with small reservoirs really benefit from a proper tune for shot count (gained at least about 15 to 20% in shot count).

To clear another thing up: at a lot of places on the internet you will find that the most efficient tune is to look for the regulated pressure at which you can reach just about 3m/s (10 fps) above your desired speed and back it down a little bit. While this is a really easy way to find a tight ES with in most cases good accuracy if you are around the right speed it is not at all efficient in terms of air consumption per shot. So, how am I so sure about this: numbers don't lie ;)

Recorded three different airguns on the bench: FX Dreamline in.22 with 20cm barrel, FX Impact in .22 with 40cm barrel and an FX Impact in .30 with a 60cm long barrel. Did the same years ago with the Steyr LG110 I had but don't have the numbers anymore.

The three setups:

FX Dreamline:

20210516_155025835_iOS.1642101145.jpg


FX Impact in .22 with 40cm barrel

20210516_175756486_iOS.1642101146.jpg


FX Impact in .30 with 60cm barrel

IMG_4749 - lbnet.1642101147.JPG


With these setups I did the exact same thing: had chosen a fixed velocity to tune on and search for the lowest pressure where I could reach the speed. This is the starting point of the graph. I chose to increase the pressure per 5 bar, retune to the chosen velocity and calculate the liters of air per shot and shot count at each setpoint.

shot count FX Dreamline.1642101498.png


shot count FX impact.1642101498.png


Maximum shot count impact 30.1642101498.png


With longer barrels the effect is a bit more gradual and you're already shooting more efficient compared to short barrels. With the .22 impact I should have included about 95 bar already but it's not in the graph, at that pressure I was really on the edge of making it to the desired velocity.

What do the graphs tell us? The higher the pressure the more efficient it shoots in terms of air consumption (liters of air per shot). This can be explained by the relation between pressure and force: the higher the regulator pressure, the higher the force on the projectile which allows it to accelerate faster (up to a certain degree, law of deminishing returns). The graph for shot count however tells a slightly different story, maximum shot count is achieved at a pressure around 20% higher than the minimum required pressure to reach the velocity.

FX Dreamline: 135 bar / 110 bar = +23% higher pressure -> 45 shots / 25 shots = 80% extra shots

FX Impact .22: 115 bar / 95 bar = +21% higher pressure -> 79 shots / 56 shots = 41% extra shots

FX Impact .30: 120 bar / 100 bar = +20% higher pressure -> 65 shots / 54 shots = 20% extr shots

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How to do this test yourself you may ask: Fill the gun to a fixed pressure (I used 200 bar) and leave the hose attached without depressurizing it, shoot a magazine (28 shots in this case) and see how far the pressure has dropped. Divide that pressure drop by the amount of shots and you know how much bar / shot is used. Multiply that number by the metric volume of the reservoir (0,3 liter in this case) and you know how much liters of air are consumed per shot. To calculate how many shots you have you take the fill pressure, substract the regulator pressure from the fill pressure and multiply it by the reservoir volume -> this gives you the available amount of liters of air to shoot with. Divide the amount of available liters of air by the consumption of liters per shot and you know the amount of shots you will have. In formulas:

(200 bar – end pressure) / magazine size = bar per shot

bar per shot x volume of reservoir in liters = liters per shot

(fill pressure – reg pressure) x volume of reservoir in liters = available liters of air

available liters of air / liters per shot = amount of shots

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Does this also work for other brands than FX: yes it does! Tried this on multiple airguns and always found very similar values. What I do see as a difference is that with lightweight internals like hammer, hammerspring and valve spring (FX, Steyr and many more) you wil barely get the increase in velocity at the end of the shot string like described in Bob Sterns article: link to the article. With very heavy and overdesigned internals (typical in older designs) you will see it at the end of the curve.

What is the impact on accuracy: the 20% increase in pressure might not be the optimal for your precision node with the smallest groups but if you look at the graphs you have at least a 5 to 10 bar window around the peak point to find a node. I have always been able to find a node within this 10 bar which gave me the precision I was looking for (which of course is the best achievable precision 😁).

Feel free to jump in ;)

Tom - @longrange_airgunning
 
Thanks for the interesting summary of your work. It's obviously a labor of love. I just try to get a balanced tune that yields a relatively long and flat sweet spot at the desired velocity. Beyond that, I just load and shoot and don't count anything. Not to say you effort is wasted, quite the contrary, I'm just too lazy to do it. Good work!
 
Interesting, and thanks for the data. I think one thing that wasn't mentioned, but I'm sure you considered it, was FPE. For each of your strings, you keep the gun shooting the same ammo at the same speed, correct? That's about the only way to compare apples to apples with your method. Another common way to measure efficiency is to calculate FPE/cu-in of air used. The Sykes calculator online is an easy plug in app for doing that.

http://calc.sikes.us/1/
 
Interesting, and thanks for the data. I think one thing that wasn't mentioned, but I'm sure you considered it, was FPE. For each of your strings, you keep the gun shooting the same ammo at the same speed, correct? That's about the only way to compare apples to apples with your method. Another common way to measure efficiency is to calculate FPE/cu-in of air used. The Sykes calculator online is an easy plug in app for doing that.

http://calc.sikes.us/1/

For each setup I indeed used a certain pellet and kept it at the same velocity and finetuned it to a ES of max 2 m/s (6 fps).

This efficiency was just meant to show that higher pressures are more efficient than low pressures in terms of air consumption per shot. The efficiency you mention is about the efficiency of the whole system in combination with a certain pellet and muzzle velocity. I’ll run the numbers soon to check what it says :) will have to check the formulas though, not really used to using imperial units :p
 
I think I've only heard the "max speed minus 10fps" (we could call this 99%) tune once and immediately ignored it since it was intuitively too close to the max.

Having not done the above shot count testing, but merely going off of accuracy, sound, and velocity ES, I ended up in the 95-96% tune range for my 700mm .30, which would be about 38-47fps under max (using a target tune of 900fps). I do remember noticing the sound starting to go up even in the 96-97% tune range, which suggests at that point it would be starting to fall down the front part of your curves. Repeating here what I mentioned in the other thread, knowing that my .30 M3 gets about ~2fps increase for every 1 bar of reg pressure that would put my tune near the peak on your shot count graph.

Thank you for posting that .30 data, because now I am not tempted to repeat your test and use up a bunch of pellets and air! I think with the 200mm, 400mm, and 600mm graphs we have a good pattern. The next question I have - what would be the best rule/method for getting to that peak efficiency, that will work in the widest variety of PCPs? Do we use %bar or %fps methods? If going with %bar I wonder if we would run into trouble on guns with small plenums?
 
I think I've only heard the "max speed minus 10fps" (we could call this 99%) tune once and immediately ignored it since it was intuitively too close to the max.

Thank you for posting that .30 data, because now I am not tempted to repeat your test and use up a bunch of pellets and air! I think with the 200mm, 400mm, and 600mm graphs we have a good pattern. The next question I have - what would be the best rule/method for getting to that peak efficiency, that will work in the widest variety of PCPs? Do we use %bar or %fps methods? If going with %bar I wonder if we would run into trouble on guns with small plenums?

both will have their limitations at some point if you ask me. Some regulators don't have a large adjustment range so you will run into that at some point (if you don't know how to restack them). Similar thing with m/s or fps if you run into the maximum energy capabilities of an airgun. For most airguns, if you stay away from their maximum limits, the %bar and %m/s / %fps will be very close so either will be fine.
 
Hey Tom,

I just got a .25 700mm M3 and was expecting to tune it the way most youtubers were tuning the mk2's (what TDK said). After watching some M3 tunes they seem to be "set it to this and you good", or "this is the most efficient tune". I asked FX what they recommend for my gun and this is the tune the suggested for FX/JSB pellets:

reg1 - 150

reg2 - 110

hammer macro - 16

hammer micro - 3.25

valve adjuster knob - 4

When I tried to tune the gun the old way I brought the 2nd reg down to 85 bar and the gun was still shooting the 25.4's at 930 FPS (max). Does this reinforce that FX's suggestion lines up with what you are saying? Using a higher reg pressure and then using the hammer spring (Macro/Micro) to bring the speed down?



Another question I had, was around setting your valve and 2nd reg for multiple projectiles

I guess if you try something like this only one projectile will be very dialled in and the others will be sort of: 





https://www.youtube.com/watch?v=D_zH118NdbI








 
Tom, Thanks for sharing the results of all your hard work. I’ve got a 22 FX Impact M3 coming in a couple days. From your information above it seems like the second regulator should be set above 100 bar. What do you think would be a good starting regulator pressure for this gun?

If your taking shotcount into account look for the lowest pressure you can get the pellet up to speed and increase the pressure by about 20%. I expect it to be around 100 bar indeed but the .22 barrel I've tested is only 40cm long, I guess you've bought one with a longer barrel and the results with different barrel lengths can't be projected 1:1 but feel free to share it so other people can use it too :)
 
Hey Tom,

I just got a .25 700mm M3 and was expecting to tune it the way most youtubers were tuning the mk2's (what TDK said). After watching some M3 tunes they seem to be "set it to this and you good", or "this is the most efficient tune". I asked FX what they recommend for my gun and this is the tune the suggested for FX/JSB pellets:

reg1 - 150

reg2 - 110

hammer macro - 16

hammer micro - 3.25

valve adjuster knob - 4

When I tried to tune the gun the old way I brought the 2nd reg down to 85 bar and the gun was still shooting the 25.4's at 930 FPS (max). Does this reinforce that FX's suggestion lines up with what you are saying? Using a higher reg pressure and then using the hammer spring (Macro/Micro) to bring the speed down?



Another question I had, was around setting your valve and 2nd reg for multiple projectiles

I guess if you try something like this only one projectile will be very dialled in and the others will be sort of: 



https://www.youtube.com/watch?v=D_zH118NdbI

I haven't shot .25 in years so not really a reference for that 😅 Yes it's pretty aligned, using a higher pressure is more efficient and you can turn down the speed with the hammer spring adjuster.

I've set my first reg at 190/200 bar and never touched it again on the M3, that reg is just to help step down from the 300 bar fill rate (only if you have a 300 bar bottle of course). Airbuks gives a good example there in that video, if you're lucky you'll have two tunes which work very well but that's a matter of trying.
 
I haven't shot .25 in years so not really a reference for that 😅 Yes it's pretty aligned, using a higher pressure is more efficient and you can turn down the speed with the hammer spring adjuster.

I've set my first reg at 190/200 bar and never touched it again on the M3, that reg is just to help step down from the 300 bar fill rate (only if you have a 300 bar bottle of course). Airbuks gives a good example there in that video, if you're lucky you'll have two tunes which work very well but that's a matter of trying.

I emailed FX USA and asked them what the fill pressure on the new M3 were because the manual says 300 bar, the gun says 250 bar and the bottle has 310 bar on it. They got back to me and said "your rifle should not exceed 250 bar." I remember when the M3 was first announced that it was suppose to be a 300 bar gun, probably why the manual says 300 bar. Not sure why they changed it.
 
I emailed FX USA and asked them what the fill pressure on the new M3 were because the manual says 300 bar, the gun says 250 bar and the bottle has 310 bar on it. They got back to me and said "your rifle should not exceed 250 bar." I remember when the M3 was first announced that it was suppose to be a 300 bar gun, probably why the manual says 300 bar. Not sure why they changed it.

The M3 is 300 bar compatible only if you have the 300+ bar gauge, 300bar working pressure bottle and valve on it. Standard it is supplied with 250 bar gauge, bottle and valve in the bottle so therefore it is a 250 bar airgun. That 310 bar is not listed as working pressure, very carefully look at what is mentioned with these pressures since some are test pressures, some are maximum pressure at maximum temperature (so that when it cools off it is within spec again) and some are operating pressure.