First, I'd just like to thank my handful of awesome customers; especially the guys who helped make this project possible. Too many times, we hear about deadbeat customers and deadbeat tuners, without stopping to acknowledge the many business relationships that enrich our sport.
Years ago, before I could afford a PCP, and before there was an affordable PCP, I started working on springers with some success. Unfortunately, they were somewhat limited for me by their sensitivity to altitude and general finickiness and hold sensitivity. Eventually, I could just afford a good PCP and it was pretty well game over for the fussy springer.... Well, not exactly. Shooting a PCP gave me an epiphany. Why should I have to work so hard to enjoy a springer? Shouldn't the springer work for me and not the other way around? With that enlightened attitude, I developed a whole new approach to soft tuning springers to make them worth owning even after investing in PCP's. The side benefit was that some were also less sensitive to altitude changes.
This project started out of curiosity. I live in Colorado and hunt, most recently, from 4,700 ft to 10,000 ft +, but then became even more important. You see, by posting about my specialization in 12 ft-lb range tuning, I started attracting FT guys. Problem: that meant that I had to tune to spec at sea level, not just somewhere around 12 ft lbs! Well, how would I ensure that my 12 ft lbs at 5,000 ft = 12 ft lbs at Sea Level? As with all of my projects, it took me a few years to gather the necessary equipment and to set aside the time to complete all these tests. I had previously performed random tests, but until I bought a GPS, I did not have a reasonably accurate estimate of the elevations for the velocity tests. So, finally, everything came together this year. This research has made me a better tuner and I hope it will give you guys some guidance in your decision-making.
In case you don't feel like reading the whole thing, below is a list of general conclusions. Here's the gist:
1) There is no single rule of thumb to estimate fps velocity drop for all springers, but there are some % drop ranges you may find useful.
2) It is important to reduce velocity drop when tuning a springer for altitude, because change in velocity will affect your POI. Unfortunately, I did not test any single gun model in multiple calibers. Another approach is to look at the drop in power, and then calculate the drop in velocity. You can examine the data below and decide which trend would be best-fit estimate for your gun/caliber.
3) Tuning for power and tuning for altitude sensitivity are opposing goals, but a soft tuned springer will not necessarily be less sensitive to altitude change. The analogy (conceptually not technically) that could be made is it's like the difference between tuning an unregged PCP for max power output or max shot-to-shot consistency. You are either trying to get the most power from the highest available volume and pressure (sea level), or minimize velocity drop as the pressure drops (climb in altitude).
Using a Garmin Rhino GPS with downloaded Topo map, and an F1 Chrony, I drove up into the mountains and tried to find at least 3 safe and level test locations. There were many variables, which I could not control, but I hoped that consistent procedures would result in reasonably valid results. I sample weighed the pellets to confirm factory specs. Each gun was tested at all altitudes on the same day. 5 warm-up shots were fired before shooting 10 shots over the chrony. Velocity was then averaged and rounded to the nearest 5 fps. Pellet selection was based on inventory at hand, while omission was based on poor accuracy performance (ex: CPs were not altitude tested in HW50S). Once I entered my data into Excel, I generated trend lines to give me some approximation of what the power and velocity should be at untested elevations.
Testing and results
The 2-scale graphs do not have corresponding scales (ex: 770 fps is not 9.5 ft lbs). They are just superimposed to show more data on one chart.
CZ 634 .177
This gun was tuned to factory spring power output, but using a much softer R7 spring. You can search the forum for one of my posts on tuning this gun.
Beeman R7 (HW30S) .177
This was a customer gun. It came to me with a stiff .120" wire and 0.520" coil ID spring installed. I tuned it with JM's Universal Mini Spring with 0.105" wire and 0.560" coil ID, which is even softer than a regular R7 spring. (The smaller the wire and the larger the coil ID the softer the spring.). I wanted to increase the power and reduce the cocking effort of this gun for my customer, which is what you see in the graph. Something else you see is that in received condition it was less altitude sensitive than post-tune. I wanted to show this as an example of a tune not designed for altitude, but for better performance closer to sea level.
Beeman R9 (HW95) .20
The tested gun was soft tuned to 12+ ft-lbs using JM's R6 spring. This gun has lately been one of my primary hunters. Its altitude sensitivity is low, and I thought about improving it further. In the end, I left the gun alone, as I am very happy it.
This was another customer gun. This gun was factory condition. Unfortunately, in received condition the gun dieseled way too heavily to get good data. Customer tune spec. was 11-12 ft. lbs., so I re-tested after the tune. As it turned out, I overshot 12 ft-lbs just a hair. We got the rifle shooting to spec, but I did not have a chance to re-test it.
Beeman R1 (HW80K) Carbine .177
This gun was tested in factory condition. So, here you go. If you want a factory gun not sensitive to altitude, this is what it takes. Clearly, in factory condition, this gun does not even come close to the potential of its potent powerplant, but it also means that there is no practical altitude sensitivity. I had to run this test twice just to be sure, and replicated the results. The large compression volume of this rifle alone does not explain the results. Since this is the only rifle I tested with a 30mm compression tube ID, the rest being 25mm and 26mm, I wonder if that is the major contributor. OH NO!!! Now I MUST buy another gun in the name of science!!!! To verify the chamber diameter contribution Ill need to get an HW35, which has same chamber diameter but shorter stroke, and, therefore, lower compression volume. Ill keep you guys up to date.
Air Arms TX200/ProSport .22
I wanted to conduct this test with the ProSport alone, but it was in pieces for refinishing for a long time. Since the ProSport and the TX200 have identical powerplants, I substituted TX200 for initial tests. I first tested a factory condition TX200. I then tuned it close to factory power, but with a much softer JM R6 spring. Based on this test and my R9 test I knew I needed a softer spring for my ProSport altitude tune. I used one of three JM R8 springs I had. Unfortunately, I found out that JM has discontinued these springs.
I first wanted to use an R8 spring, but found it has been discontinued while working with the ProSport. I tuned it with the Universal Mini Spring, using what I had learned from all other tests, and it worked out far better than my expectations. The spring I used is just about maxed out here, and JM just does not have the demand to make a large variety of soft springs. It is now my primary cross-altitude springer.
Here are some graphs of overall % drop per 1,000 ft of elevation gain. I hope these numbers will help you guys draw some general conclusions.
Clearly, many more tests that could be performed, but time, weather, and finances all conspired against me. I now have some ideas, and will continue to keep y'all up to date. I'll be taking on more customers starting January 1, so let me know if you are interested in my work etc. I do have 2 discontinued R8 springs left for those interested in a TX200, ProSport, R9, HW77, HW97 altitude tune.