tuning a pcp air rifle---how

[Little Doc]

I read that people tune air rifles. what does this involve.
I would like to read what the process is and what is expected to accomplish. all shots to near same velocity I would guess.
anyone know?
m

[MT Chambers]

Sorry but I would let the pros do it for me, it involves work on the valve and/or the hammer/hammer spring....it can be done to get more consistant velocity and/or more shots per fill, although regulators are meant to do the same thing. Presumably one could also tune for more velocity, at the expense of "shots per fill"....it's prolly diff. for every make and I won't play around with my Wolverine, because of the old adage "what can go wrong, will".

[rsterne]

Tuning a PCP, in the simplest sense, means using a Chrony to determine the ideal fill and refill pressures that will produce an extreme spread of velocities that is acceptable to you for the job at hand.... Any unregulated PCP (most of them) when tuned properly, start out a small amount below the peak velocity, the velocity increases as the pressure drops, plateaus, and then starts to decline.... The trick is to start and stop your shot string when the velocity is the correct amount below the peak.... As a rough rule of thumb, at 25 yards, you can get away with about a 10% Extreme Spread (ES) in velocity without it increasing your group size significantly.... At 50 yards, that shrinks to about a 4% ES (what I use for most guns), then to about 3% at 75 yards, and if you are shooting at 100 yards, you will want the ES to be 2% or less.... A good regulated gun will hold less than a 1% ES with sorted pellets (sorted by weight and diameter) but straight from the tin 1-1.5% is more common, sometimes as much as 2%....

If the ideal fill pressure is higher than what your gun allows (or higher than you want to fill to), then you have to reduce the hammer strike to drop the velocity of the first shots.... Any time the first shot is the fastest and all subsequent shots are slower, your gun either needs less hammer strike, or filling to a higher pressure (providing safe to do so).... The hammer energy and momentum can be reduced by reducing the preload on the hammer spring (the most common method), reducing the travel (stroke) of the hammer (the Marauder has that capability), or lightening the hammer itself (usually reserved for those with a machine shop or access to specialized custom parts).... Some of the Korean guns prevent the hammer from opening the valve fully by using a power wheel that mechanically adjusts a hammer stop, but this generally leads to a "first shot fastest" shot string when the power is reduced because it takes away the "self-regulating" affect when the common "knock-open" valve has in an unregulated PCP....

Beyond simple adjustments, tuning for more power generally either means increasing the fill pressure (if safe) accompanied by a stronger hammer strike (to open the valve against the higher pressure).... or by modifying the valve and airflow path so that more air can move at the same pressure.... Some guns, like the Marauder and Air Arms, also have a restricing screw in the airflow path that allows you to detune the gun without affecting the fill and refill pressures, a nice feature.... If you aren't familiar with the basics, delving into modifying the valve and porting is best left to the experts....

Bob

[Little Doc]

thanks Bob
this helps. I know nothing about this. new to pcp. not sure I want to tackle any of this.
have a daq 308 and fill to 3000 as it is recommended. chrony says velocity drop from shot to shot for three is 100fs for each. someone suggested to increase fill by 100 and chrony till first two shots were same velocity. have not had a chance to do that yet. shooting at 6in diamond swinger at 120 yds will miss hit miss for three shots with same hold. looking to see what needs to be done to change this to three hits. your explanation helps me understand.
thanks
m

[rsterne]

I have a DAQ .308 Exile, and stock it shot 3 shots of 836, 789 and 736 fps, and dropped from 3000 psi to 1800.... This is a classic string for a PCP with (IMO) too much hammer strike (for the fill pressure).... I removed the hammer spring, slid it over a dowel and compressed it until all the coils touched.... When released, it was nearly an inch shorter (4.50" instead of 5.44").... With no other change, I got 5 shots at 833, 832, 824, 811, and 792 fps, and still had 2050 psi afterwards.... Note that the 5th shot was faster than the second shot the way I received the gun, I got 2 shots nearly as fast as the first shot stock, and I got 5 shots instead of 3 while using less air.... To me, this was a no-brainer way to tune the gun.... and the cost was zero.... BTW, I didn't compress the spring fully all in one shot.... I did it a bit at a time, checking the velocities as I went.... As it turned out, fully "setting" the spring worked for me.... In your gun, it might be too much (or too little)....

As you state, you could also increase the fill pressure, and eventually the first two shots will be about equal velocity.... At what pressure that will occur I have no idea.... it could be 3300 psi, 3600 psi, or 4000.... I do however, know that the instructions that came with my Exile stated a fill pressure of 3000 psi.... I tuned the gun for a good shot string while staying with what I know to be a safe fill pressure....

Bob

[Little Doc]

thanks bob
this is something I will try. worst case could have to get a new spring. not a quote but I think DAQ states on his web site test pressure is 9000psi and burst pressure is 12000. don't remember I may have got this info elsewhwere. I would feel ok with some increase in psi. have tried to follow mfg. recommendations and do not plan to fudge much.
what bullet do you find works best in your 308?
m

[rsterne]

So far, it likes the Lee 113 gr. FN.... PCPs are typically designed with a 3:1 or 4:1 safety margin, if you fill to a higher pressure, that margin shrinks.... Dennis knows what he's doing, but if he says 3000 psi I would tend to believe him.... Filling beyond 3000 would be a challenge if you are using a pump or a SCUBA tank, no issue if you are using a 4500 psi SCBA tank or a ShoeBox compressor....

Bob

[melloairman]

thanks bob
this is something I will try. worst case could have to get a new spring. not a quote but I think DAQ states on his web site test pressure is 9000psi and burst pressure is 12000. don't remember I may have got this info elsewhwere. I would feel ok with some increase in psi. have tried to follow mfg. recommendations and do not plan to fudge much.
what bullet do you find works best in your 308?
m
Doc I am glade Bob has reaffirmed every thing you were told in your first post about your DAQ . Keep us posted as to what the out come is .GL.Marvin

[Little Doc]

I am not interested in hotrodding an airgun. looking for consistency from shot to shot. I am not getting that now. trying to get an understanding of how these thing work and what makes them work better. I would like to get a number of shots to shoot to the same place.
All info and advice here is greatly appreciated. My thanks to all.
m

[melloairman]

I am not interested in hotrodding an airgun. looking for consistency from shot to shot. I am not getting that now. trying to get an understanding of how these thing work and what makes them work better. I would like to get a number of shots to shoot to the same place.
All info and advice here is greatly appreciated. My thanks to all.
m
Doc I am the same way . I detune most of my rifles . Both Bob and I have told you to put a set in the spring . If you do not have the time or are uncertain as to how to get the spring out . Do what those that never put a set in their springs tell you not to do . No bullet but chock the rifle and let the rifle set for 24 hours with the spring compressed . Then chrony it again . This should put at least a parcel set in the spring if not a complete enough set that things should not change rapidly or at all . GL.Marvin

[Little Doc]

87471
hope the pic got attached.
If it did it is 120yds. first shot is at 9 oclock just left of center second shot is at 1 oclock and third shot is at two. all shots were held to center. first is 4 mill dots holdover, second is 5 and third is 6 milldots. zero is 50yds. Shot several over the chrony.

1. 950 1. 957 1. 961 1. 958
2. 909 2. 907 2. 907 2. 901
3. 864 3. 866 3. 851 3. 870

each set was filled to 3000psi and refill was at 1700psi

wanted to compress the hammer spring as suggested but not able to remove the rear plug. even tried channel locks with leather belt to protect. could not move. any ideas to do this without ruining the finish and chewing up the knurling.
would like to get these three shots near the same place.
by the way the bullet is 92 gr.
m

[Little Doc]

correction. first shot is at 7 not 9.

[rsterne]

You did remove the rear breech screw, right?.... It passes down through the bolt plug and threads into the rear tube plug.... There is an index mark on the bottom of the tube plug so that you can get the orientation correct to get the breech screw back in again.... IIRC my rear tube plug was only finger tight, no tools required (once I removed that breech screw)....

Bob

[Little Doc]

rear breech screw??? I had no idea that that screw held the bolt plug in place. I did see the index mark. Thanks I will give that a go tomorrow.

[rsterne]

It goes down from the top, through the bolt plug and the main tube, and threads into the rear tube plug to hold the breech down at the back....

Bob

[Little Doc]

Yesterday I disassembled the rifle to get the hammer spring out. fully compressed it, reinstalled and shot several three shot strings. no change in velocity. the spring measured 5.050 before and 4.960 after. will have to get more air before anymore testing. Bob how long did you hold the compression on the spring. will hold time make any difference and is it critical. Hope to get air today. more tomorrow hopefully.
m

[rsterne]

Springs basically "set" or don't set as soon as you compress them.... Yes, they may eventually relax further, depending on material and coil count, but 99% of the set will occur immediately.... I pushed the spring completely coil bound over a dowel (to keep it straight) 2 or 3 times and then measured it.... It went from 5.44" to 4.50" once I set it completely.... As I said, I didn't know how much effect there would be from doing this, so I went easy, maybe 1/4" at a time, but ended up setting the spring completely....

If your spring only changed 0.090", this method isn't going to work for you.... you will have to cut a coil or two off to get what you want.... I would go about 1/4 coil at a time and Chrony each time until you get what you want.... IMO, the ideal string is 3 shots within just a few fps, and then a gradual fall-off.... To get that, the first shot will have to be a few fps less than the second shot.... Some guys figure that the best way to tune a big bore is for two shots of equal velocity and then for it to start dropping.... and other just go for the maximum on the first shot (the way your rifle is now) and to heck with the air consumption.... I think they would be surprised how little they would lose in velocity by tuning for 2-3 shots while using a lot less air, but to each his own....

One thing you should be aware of.... If you are thinking of using heavier bullets, you should really decide on a bullet before going too far with cutting your spring.... The heavier the bullet the more hammer strike you will need.... If you used 150-170 gr. bullets (if they would even fit the chamber, just giving you an example) you may find that the way your gun is tuned now would be just fine.... You might get 3 shots in the mid 700s quite close in velocity with the spring the way it is.... As I said, my gun likes the 113 gr. Lee flat nosed design #C309-113F, it shoots that around 830ish (180 FPE)....

Bob

[Little Doc]

Bob,
you keep being more helpful, thank you. your last comment about bullet weight is what I need to do before any spring cutting. I have the 92 gr, a 130gr, and a 150. I have shot all in the gun but only to see if they would work. I have a 165 and a 173 that I could try also. this may be the place I need to start before I consider the spring.
if I knew where to get springs it might be interesting to find the right combination for each bullet. Got air today and may try other bullet weights before anymore effort to tune. Thanks again for sharing your knowledge.
m

[rsterne]

Let me know at what point the bullets get too long to fit the chamber (if at all).... I've never tried anything longer than a Lee 120 gr. RN.... As you increase the weight the FPE will likely increase slowly, but the velocity, of course, will drop.... At some point, the arc in the trajectory will become more important to you than any small increase in power.... Keep records and let us know what velocities you get with each bullet weight and if the shot string evens out.... We can all learn that way....

What is your intended game?....

Bob

[Little Doc]

No game is legal to take with an air gun now. Hogs are plentiful and no problem. looks like my game will be ringing steel as far as I can see it. looks like steel targets for now. will post results with the heavy bullets.
m

[melloairman]

No game is legal to take with an air gun now. Hogs are plentiful and no problem. looks like my game will be ringing steel as far as I can see it. looks like steel targets for now. will post results with the heavy bullets.
m
Doc I hope you do not mind me replying to your post . When you first posted you were asking about a regulator set up and I figured you were needing one internal for hunting . I shoot a lot and only target at this time . Have tuned for single fills wasted a lot of air with bleed off and fought the poi shift . Then I broke down and bought a in line regulator on e bay as a matter of fact I bought 2 . One for a booster pump that I really do not use any more now that I tether . The regulator makes it real easy to change springs and psi to get the accuracy and shot count you want . Well worth the money for target work . And there was a man making power adjusters for the DAQ that would help . His name was Will Hickman . Or you might be able to talk Bob into making one for you . Here is a sample of what my .308 Corsair is doing with one of the springs I use . GL.Marvin
http://i301.photobucket.com/albums/nn55/melloroadman/308%20CORSAIR%20CHRONY%20READINGS/IMG_7179-308CORSAIR10-8-12.jpg (http://s301.photobucket.com/user/melloroadman/media/308%20CORSAIR%20CHRONY%20READINGS/IMG_7179-308CORSAIR10-8-12.jpg.html)

[Little Doc]

thanks for the reply. I do not mind. I am searching for as much info as I can find. this is my first and only experience with a pcp. I have learned much from this post and all info is appreciated. the poi shift is what I find the most frustrating. I really don't want to be tied to a tank at his time. The power adjuster I think would accomplish what I need-want at this time. would like to know how they work. I have the means to make one I think just need to know how it is made and how it works.
the regulator makes it easy to change springs and psi. I can see the psi but the springs change I do not know what you are telling me.
thanks for the info.
m

[melloairman]

If you go back to were Bob explained that a heavier spring allows a higher psi I think you will understand what is going on . Basically what I am doing is setting the rifle at the top of a bell curve at a set psi . I can low psi by lowering spring rate . Or I can raise psi with a low spring rate and cause a parcel valve lock .In other words go past the top of the curve . It is well worth being tied to a bench for target work if you really want accuracy that is repeatable time and time again . The power adjuster would be a bolt that would screw into the rear plug and press against the base that the spring rest on . Screw the bolt in and it applies pressure to the base of the spring . Screw it out and it releases the pressure . Marvin

[rsterne]

If you're into ringing steel at 100+ yards, you may tire of the trajectory with really heavy boolits.... I'd look at the 113 Lee FN.... good BC for the weight to help buck the wind.... and your gun should fire it in the mid 800s....

Bob

[Little Doc]

Bob the answer to your question about heavier bullets. Today I shot several at 50yds. the 150gr chambered without effort, so did the 170 same for a 200. the 200 is tipping at 50yds. so a no go the 170 seems ok the 150 finally gave me the first two shot together with a 3400psi fill.
After shooting several I went to the pond to shoot some turtles. well it was 0 for me. most shots were under 25 yds and a head above water was the norm. Very frustrating. as shots were over under way over etc. Friend has a Maurader 25 and can hit a golf ball with every shot at 50yds and get 20 shots to a fill. maybe I am using the wrong caliber. I guess plinking at all distances is what I will usually do with this. not really a group shooter. Still I would like to get several to shoot to same poa.

[rsterne]

The 200 gr. would be too long for the DAQ's 16" twist, but yes, the 170 gr. should be OK.... What velocity did you get with the 150 gr. and 170 gr. at 3400 psi?....

Bob

[Little Doc]

Bob I did not shoot the 170 at 3400. the 150 was 844,791,752,713. the first two shots were side by side. I think this would be my hunting bullet if I could deer hunt with an air gun.
m

[melloairman]

The 200 gr. would be too long for the DAQ's 16" twist, but yes, the 170 gr. should be OK.... What velocity did you get with the 150 gr. and 170 gr. at 3400 psi?....

Bob
Bob when I talked to Dennis he told me that with the .308 he varies the twist rate . That not all of his rifles have a 1 in 16 . It depends on what the customer tells him the weight of the bullet he will be shooting will be .As to what twist rate he gives the barrel . Marvin

[Little Doc]

I do not remember being ask or given the option of bullet weight. Must have missed that. When I ask about bullet weight I was told that the guns were made to shoot 130gr. bullets. I bought a mould from NOE that looks like your avatar Marvin. I can not find that either the 92,130,150, 170 shoot any better that the other with a 3000psi fill. When I get a chance I think I will use a tight patch and see what the twist is.

[rsterne]

You're still getting a very large velocity spread even with the 150 gr.... If it were my gun I would either install an RVA to allow me to back off the hammer spring preload, or I would start clipping the spring a quarter coil at a time.... BTW, that is a LOT of power, 237 FPE, I'm impressed.... I'd be happier with 2-3 shots at 208 FPE like your second shot, however....

Interesting.... I was never contacted to ask me about twist either.... Apparently this is not a service he offers ALL his customers?.... He certainly doesn't mention it on his website, in fact his website says that the first you will hear from him is when your gun is ready to ship and he wants to be paid, and that was certainly the case for me....

Bob