Hey Chris, well, you have asked about 50 questions in that couple of paragraphs, LOL. The best answer is to go to the PCP gate and look at some of rsterne's many, many, posts about tuning PCPs. Your comment about the intrigue growing hits close to home. There is an incredible amount of science in PCPs for those who wish to go there, but it can also be pretty much ignored if all you want to do is shoot. The breadth of the hobby can be whatever you want it to be.I am not familiar with the PBBA airguns, so my answer will be rather general. First suggestion might be to call the manufacturer and ask if they have a de-tune kit for your gun. They should have a record of spring, transfer port, hammer weight, etc, for your gun and should know what to do.You are getting over 200 FPE from a .308 and that is definitely in the powerful hunting gun category. The valve probably has a big throat with a heavy spring and big transfer port, all for the purpose of flowing plenty of air. You said you got a slight bell curve with the 150gn bullets, but with lighter bullets, your data shows that the velocity just drops off with the tank pressure with no bell curve. The big throat in the valve makes it hard to open the valve against the tank pressure. The fact that you are loosing the bell curve means that you aren't really fully opening the valve with the lighter bullets. Lightening or shortening the spring won't give you a bell curve but will drop the velocity, and will also reduce the pressure at which the hammer becomes incapable of opening the valve, again, without a bell curve. There are numerous discussions about the pros and cons of light, fast hammers, vs heavy slow hammers, etc, etc. Tuning is as much an art as it is a science, with lots of trial and error, but rsterne has outlined several methodologies that make it a logical process, rather than a random approach. One thought is that drastically decreasing the diameter of the transfer port might help your situation. Your valve, with the big throat, will be difficult to throttle down, so introducing the restriction between the valve and the barrel might make tuning with the spring force and preload less touchy. Good luck on this.BTW, doing a few calcs, it looks like your gun might have a 150cc tank. Air efficiency is in the area of .8 FPE/std-cuin of air. That's not great for target shooting, but it is ok for a strong hunting rifle.I hope this rambling helps.Lloyd
To me it looks more like too much hammer energy, so too much spring. First I would try and reduce that and then see what happens. Biggest possible ports with short dwell and there hides the efficiency. IMO. Marko
You second string is much like the way my DAQ .308 Exile shot, with similar weight bullets, in stock form.... It too used 1200 psi in just 3 shots.... All I did was "set" the spring by compressing it over a guide (the coils were widely and unevenly spaced, it looked like the spring was wound by hand).... I did this a bit at a time, and each time I shortened the spring by compressing it, the shot string got better.... I ended up fully setting the spring (compressed to coil bind and then allowed to rebound), which shortened the uncompressed length of the spring from 5.45" to 4.50", and reduced the preload from 1.2" to just 1/4".... I was rewarded with a string that only lost 3 fps from the highest velocity, and had 5 shots within a 4% ES, with my 5th shot being faster than the 2nd shot the way the gun was stock.... In addition, the gun only used 1000 psi for all 5 shots instead of 1200 psi for only 3 shots.... The efficiency went up from 0.38 FPE/CI to 0.88 FPE/CI....It is unlikely you will be able to simply "set" your spring, but replacing it with one with a lower spring rate may well give you similar results.... Good Luck !!!Bob
I always take off the weakenig of spring by compressing it completely for some 24 hours and only after that cut the spring into its final length. Otherwise it changes during time. Well it still does but very much less.
Short and stiff spring also reduces hammer bounce thus giving you more shots / air used. I'd get some replacement springs and start cutting off coils after setting those springs. Add one more coil once you cut off too much
You are going to anyways but still I'll ask can you give us a shot string after each modifications? Just for the interest. Marko
You could also try to increase valve channel and port size as well (together), that may increase efficiency and/or lighten hammer but these are the ways hard to go back if it didn't work as well as shortening spring. If you take something off it is hard to return if it seemed better.There are so many variables including valve stem size and plate form that it is quite impossible to tell anything one fits all type of solution.
I always set my hammer springs before installation.... so doing that on my DAQ (especially on the crudely made spring supplied) was a natural.... I have found that once set fully (ie to coil bind), springs don't further reduce in length over time to any measureable degree after that.... at least that has been my experience.... Bob
There are so many ways of doing the tune and so many ways that valve systems are constructed that it is rather hard to give generic guidelines that work for all guns.