30 shot string

Thanks for weighing in Walt. You made some really good points there. The most important one is about efficiency (I think). One thing folks need to realize is that every additional fps requires more energy than the 1 fps that came before. That’s true even if the gun were 100% efficient. We know this because every additional fps results in more FPE than the one before and we can’t get something for nothing! Time for a little math demonstration (with no actual math shown!).

Take a 21.1gr Kodiak and drive it to 900 fps and you get 37.96 FPE.
Take the same pellet and drive it to 925 fps and you get 40.10 FPE, an increase of 2.14 FPE. So, if we go to 950, we ought to get 42.24 FPE, right? No, we get 42.29! And if we add another 25 fps, we get an additional 2.26 FPE. Going from 1075 fps to 1100 fps gets us an additional 2.59 FPE! Energy is not a linear function, going from 1100 to 1101 is a lot harder than going from 900 to 901, about 18% harder assuming identical efficiency of the gun! And, we can be pretty sure the gun is not at 100% efficiency or even as efficient at 1100 as it is at 900. None of this takes into account compressibility effect or the other losses incurred at SS velocities but we don’t need that here because we’re not trying for ultra speed in this case. What we’ve established is a basis to determine if we have a realistic goal. And, a an idea of the task should we decide to go for a string of higher energy shots in the future.

So, what we’re looking for is 40 shots at an average of 925 fps. Is that doable? Let’s look at what the gun does now: Referencing the OP string, we see 30 shots at an average of 1047.87 fps for an average of 51.45 FPE yielding a total of 1543.74 FPE. But, the power is not linear, right? Correct, the power is not linear and thus taking a simple average could throw us off. But, the velocities in this case are fairly consistent and so the error is not huge. In fact, if you take the energy from each of the 30 shots and sum it, it comes out to 1544.83 FPE. As expected, the shots above the average fps added more energy than the shots below the average detracted from the final sum.

OK, the gun right now will output 1545 FPE over 30 shots. If we get pretty pessimistic and guess that the next 10 shots would have averaged 700 fps, we still have another 230 total FPE to work with. If the next 10 shots were to average 800 the output energy would total 300 FPE and at 850 average, 338 FPE. Since we are going to be looking at a longer string of slower shots (where the gun is likely to be More efficient) it is safe to say that we have at least 1800 FPE out available…

What we want is 40 shots at an average of 925 fps. In fact, what we really want is 40 shots at Exactly 925 fps (but we got all realistic and settled for 900-950-900). 40 shots at 925 equals a total output of 1600.4 FPE, 40 at 950 equals 1691.6 and 40 at 975 equals 1782.

It all looks good so far but we have to deal with the valve dynamics. Since the gun is not regulated we have to find the balance where the inertia from the hammer opens the valve enough to let x air at y pressure out to get 925 fps and 40 shots later let the same hammer inertia let z air out at b pressure to get the same 925 fps! Since the pressure in the tank when full is adjustable we get to pick the y pressure. We also get to pick the hammer inertia.

In fact, if you (we) got really anal about it, we could adjust hammer inertia during the string to compensate for changes in efficiency. Assuming you’re not willing to get that anal, it’s time to pick y pressure and inertia. Since the original string showed a good curve, we can deduce that a lower inerti will not open the valve as much against the same initial tank pressure.

And that’s why I suggested the lower initial pressure and lower inertia, because the average velocity is going to be less but the total energy out is going to be similar. Sadly, I don’t have the numbers to start with the right combination!

It will take some experimentation and adjustment but I think 40 fairly consistent shots from 900-950 and back to 900 are within reach. Or, you can just aim higher!

Sam, Jerry’s advice is spot on. Think of it as integrating the area under the curve when you graph up velocity vs shot. But really, shot number is a dimensionless variable. What we are really imputing on the X axis is not shot count but, really, tank pressure. What we are assuming is that each shot consumes a relatively stable amount of air, so that it is safe to use the shot # as the Y variable, when that is not strictly true.

Nevertheless, Jerry’s advice is, again, spot on. You have to tune the variables to coincide with what you want in terms of performance. The tank on this gun is so huge that we are dealing deep into double digits on shot count, which is a phenomenon that does not occur with many other models of airgun. You have to keep in mind there are diminishing returns on the top end. Everyone knows about the sound barrier (1138fps at STP) and the effects that buffet the pellet as you approach the trans sonic barrier. It is not an absolute barrier, think of it as increasing turbulence and not a ‘wall.’ The military made this mistake when it first specified ammunition for its subsonic silenced guns to have velocities at 1050fps. They found that under certain conditions of humidity and temp, the trans sonic barrier was so close to this, that in the real world they were too close and some shots were too loud and unpredictable in performance. I have always selected 975fps as my barrier in Hawaii, and no more.

But there are definite physical diminishments of the return you will receive in terms of external ballistics once you pass a certain point. In other words, it is not a straight line association between air used and pellet velocity. The higher you go, the more air you will consume, and the faster the rate at which this association changes. The AF guns are particularly susceptible to this due to their small constrictions in the valve orifices. Yes, they can be modified and even new valves made for them, but they still have to fit into the frames and bottles, so you cannot make gross changes and still have a gun that will interchange parts with other AF guns. Ask Cygnus. Yes, he built a 500ft-lb ‘AF’ gun. Do you know how much AIR his .458 used per shot? When you divide out his shot number and take cumulative notice of how much lb-ft he was getting per tankful and compare it to your current setup, there is no comparison. Efficiency goes out the window when reaching for peak power.

I would not make recommendations to you because I have no idea what you are after. My circumstances always revolve around hunting because there is no FT in hawaii, therefore I am not interested in shot count; only in peak power for 3 to 5 shots. If there were FT here, I’d be looking for 30 to 40 shots with a refill halfway through. But, and this is a certainty, I would NOT be using an AF gun to shoot FT because sitting and kneeling requires some contortions in body position and the AF is particularly susceptible to flex in the valve area, plus it has unacceptably high sight position and therefore unacceptable parallax error.

But, if you are willing to listen to me, then I’d say this: Do not try for more than about 975fps, irrespective of your pellet selection. Use one pellet and stick to it, nevermind swapping ammo for different uses. Tune the gun for that 1 pellet and stick to it. 20 shots is plenty, in my opinion, but in my opinion you are a little high on velocity. I wouldn’t push that close to trans sonic unless I had a reason to, like trying to squeeze more power out of a shot in order to be able to crash through a brain case at close range, which is sometimes what I do with the .25 eun jins. I’ve pushed them 1050fps on purpose, but I never have to be more accurate than 1″ at 20 yds, so my requirements are skewed. I think if you brought it down to where Pab is, you might squeeze 30 shots in the thick part of the shot curve. Unless you are habitually shooting at super long ranges, what use is trying for 100fps more? Your pellet selection tells me you are not trying to do that. the kods are not meant for shooting at anything over 60 or 70 yds, which is longish for any airgun, although with Jerry’s stuff you can stretch it to 100yds and a tad more on a regular basis simply due to the increase in sectional density. So, if you are going to use only kods, tune it back a tad. Flatten the curve. Try to start at 925 or 950 and get 30 good shots.

Check the o ring in valve body that clenches the stem. Mine has a strange bind, I’m convinced the CA glue they slobbered all over the valve when they installed it reacted with the buna N rubber and killed its characteristics. But I haven’t bothered to find replacements more suitable so have not changed mine yet (I think trying polyurethane or viton rings might yield superior results due to greater lubricity…but shorter life).

my 2 cents. or 3.

walt

I’ve seen. Very shiney!
Seems to make a pretty big improvement to say the least. Hopefully it will give you the adjustability you are looking for. Though if it is shooting good, no poi change out to 100 yards, then your just being a geek like me.. 😆

I’d say yes. Mine does 940 to 980 to 940. Thats over 30 shots in .25 using Kods. And I start at 2550 ish. But again, thats using a blod valve so I don’t know about the HiFlow or power wheel. No reason you can’t tune down the TT to those sort of numbers. If hes done the valve work etc, should be doable?? As to consistancy? Maybe it will make the difference. I would be inclined to try….. 😀

But I am spoilt with my valve…. 😎 😆

Nah Dude, tinker, tinker, tinker…….. 😆