regulated pcp accuracy tuning - finding optimal fps

[mschlee]

been reading a lot of the technical stuff on here and ham. has helped me begin to understand the tuning approach, but i have limited practical experience.  a few questions i have that i cant seem to find answers to. i'll recap my understanding of the process and pose questions at end:

(over)simplified tuning process
1. begin with selecting the projectile you want to shoot
2. calculate the fpe required to shoot the projectile at the appropriate fps ("x") to ensure your gun can achieve that fpe
3. start with hammer spring about midpoint
4. start with reg set at max
5. increase pressure to the gun with filling apparatus about 10 bar at a time until you reach an fps of "x" + some headroom ("y")
6. if velocity starts to plateau before you reach "x+y"  start again at a higher spring setting
7. once you hit the right speed (x+y), write down that pressure, degas and set your regulator to that pressure
8. fill the tank with reg set at new pressure and start backing off the hammer spring until you hit x

i believe this will put you right on the "knee" as desired. if i'm doing something wrong please let me know.

questions:
1. how do i determine "x"? articles on tests seem to say pellets are best between 850-950fps but that is a pretty wide spread. for slugs, i see general guidance that they need higher fps, but no specifics.

2. what is the best headroom ("y") i believe somewhere in the 3-5% range based on graphs in the ham articles but would like to confirm.

3. related to question 1, given every gun is somewhat unique, if my "x" is not perfect for my selected projectile, how should i go about finding the x that provides optimal accuracy in my gun? i dont think i can just play with one of the variables (reg pressure or hammer spring) because they need to work together, but it seems extremely time consuming to have to retune through the above process at different "x"  targets till i find the optimal setting.


thanks in advance for any help others can offer - appreciate all the knowledge sharing on here as i'd be nowhere without it.

[ranchibi]

Michael, you are tuning correctly! The thing about tuning is also about accuracy. You guesstimate what speed you want to shoot a certain projectile at then you set the reg pressure approximately and adjust HS until you hit the knee then about 5% below that with HS adjusted. Then you check accuracy as each barrel with each platform will probably be a little different. You may need a stronger spring or a lighter one in tandem with reg pressures. I set my RAW up to shoot 18gr pellets @ 950fps (usually OEM tune is 950fps for 25gr .22) with reg set @ approximately 140 bar with 2 coils cut off of HS. it is very accurate in this configuration. It is also not as efficient as it can be in this configuration getting close to 80 shots on a 240 bar fill but the accuracy is just so consistent. It also is very accurate slinging those 18gr pellets @ 1050fps down to 880fps but not what I want/need for shooting almost daily and pesting @ long ranges (ground squirrels and starlings). The reason I like this tune is I can also shoot 17.5gr slugs @ 935fps accurately for pesting and hunting. So, your tune should fit your wants/needs. I have a hammer de-bounce device that makes it way more efficient even lowering the reg set point to 120 bar almost doubling my shot count @ same speeds but my ES grows from 2-7fps to double digits and that is a pet peeve of mine, ES! As you know, the lower the ES the more consistent groups you should get all things being equal. You know how to tune, just tune for your wants/needs with accuracy.  Happy New Year 🎈!

[mschlee]

Thanks Ranchibi, and happy new year! that is helpful. where i get confused is after i tune on the initial fps target and reduce the hs to get the knee, i'm in the right ballpark for accuracy, but then what do i do to start evaluating accuracy?

example may help. say i set up shooting 25 cal exact kings at 930 fps. it shoots 1 inch groups at 50yrds but i want to see if i can get more out of it. what do i do? tune for 940 and test accuracy, then tune at 950 and test accuracy...wash, rinse, repeat at various points until i hone in on a good group?

feels like that is a lot of tuning (lots of air, pellets and time - particularly if i dont have an external reg adjustment). i was thinking i cant really just adjust hammer spring cause that is going to throw off the tune - need to adjust reg and hammer at same time.

maybe it's just wishful thinking and only way to do it right is to re-tune but better to check with experts before i start down that road.....

[JimD]

That's an interesting way of doing it but it is not what I do.  My P35-25 regulator acted up, threw the first shot at lower velocity, until I turned the hammer spring up to about the point of highest velocity for it's regulator setting.  Responses on GTA indicated this was the possible problem and not super unusual.  So what I do is:

1)  Shoot at the settings the gun came with
2)  Turn up the hammer spring and see if velocity increases.  If it does, keep going until it does not.
3)  I am typically shooting groups while I am doing the first two steps but I will also shoot groups with up to 1/2 turn more hammer spring.  In my Avenger, a little more hammer spring significantly reduced group size.  It has not helped my other guns.  If the gun doesn't care, then I will set the hammer spring at or slightly below the point of maximum velocity.  I do not find that I can vary velocity very much with the hammer spring setting.

If velocity is too high for the projectile I start with (for me, much over 900 fps, I want 800-900) I go to a heavier projectile.  If under 800 I go to a lighter.  My P35-25 was a little high, I thought, well over 900 fps but accurate with H&N FTTs (20 grain) so I turned the regulator down to get them going about 875 fps.

On my Avenger, I degassed it immediately after getting it because I knew the regulator would be set really high.  I turned it done to the minimum and refilled the gun.  I then shot different projectiles and increased velocity with the reg setting.  I increased HS when necessary as I raised the reg setting but the hammer spring in this gun is overly stout in my opinion so it was set at the minimum for the lower reg settings.  If I really wanted to use those lower velocities I would have clipped the spring or put in a lighter one.  Shooting or degassing to a minimum pressure would have made sense for what I did on my Avenger.  Then I could have filled it gradually up.  That would have told me velocity as a function of pressure but I couldn't have tested a variety of pellets at each pressure level. 

I think it's fine to have an idea of what pellet you want to shoot but I have had no success changing velocity or otherwise tuning the gun to shoot a particular pellet well.  In my experience, my guns tell me what pellet they want, I cannot really tell them what to shoot accurately.  My initial testing is to sort out what pellet or pellets the gun will shoot accurately and also, at the same time, determine where the regulator and hammer spring should be to push those pellets 800-900 fps. 

[ranchibi]

Thanks Ranchibi, and happy new year! that is helpful. where i get confused is after i tune on the initial fps target and reduce the hs to get the knee, i'm in the right ballpark for accuracy, but then what do i do to start evaluating accuracy?

example may help. say i set up shooting 25 cal exact kings at 930 fps. it shoots 1 inch groups at 50yrds but i want to see if i can get more out of it. what do i do? tune for 940 and test accuracy, then tune at 950 and test accuracy...wash, rinse, repeat at various points until i hone in on a good group?

feels like that is a lot of tuning (lots of air, pellets and time - particularly if i dont have an external reg adjustment). i was thinking i cant really just adjust hammer spring cause that is going to throw off the tune - need to adjust reg and hammer at same time.

maybe it's just wishful thinking and only way to do it right is to re-tune but better to check with experts before i start down that road.....

Micheal, EXACTLY! It's not rocket science but a baseline to start by. We all envision a certain speed combining accuracy from that guideline and that can happen but maybe not....LOL! And, pellets are much easier to tune for than slugs (most of the time), pellets are more forgiving being not spin stabilized (a whole other headache 😂). Trust what you are doing as it's quite clear you know how to tune a PCP!

[mschlee]

thanks Jim. so if i understand correctly, with way you tuned the avenger, you (making numbers up):
1. degas and turn reg down and leave hs in normal position (assume around midpoint)
2. pressure up to a low setting say 80bar
3. shoot a 5 shot group with a selection of pellets
4. increase pressure by 10bar and adjust the hs "if necessary"
5. shoot a 5 shot group with a selection of pellets
6. repeat 4 and 5 until about 900fps
7. select the pellet/fps that held the best groups throughout the test and tune there.

if yes, when you say adjust hs "when necessary" when is that? sorry for detailed questions but i have a feeling you are doing this a bit by feel which i have none of being new to the game so may not be as easy for me.

would it be better to just leave the hammer spring at strongest setting when doing this - that way you eliminating it from the equation and focusing strictly on the best pellet at the best speed regardless of efficiency?