Dynamic .25 cal PCP - First Impressions and Observtions

[Wayne52]

One thing I need to do before doing much more chrony work in .25 is make a better pellet trap, It's probably about time I started thinking about
that because the one I'm using now really isn't holding up to the heavy pellets that good, I need to make one with mulched rubber in it.  I haven't
really done any chrony tests with the 36 or 38 grainers.  That in itself means finding the right weight for each like I did for the 34 grainers.

For tuning my 25 I basically turned the regulator up and adjusted the hammer for each adjustment until I found the 60fpe range with the 34.5
grainers, my original goal was getting ready for 25 cal BBT's but after I got the mold and tried them I found out they needed sizing so it could
be quite a while before I get to that point.  I need to start watching Craig's list for a loading press, I use to have one but it's gone.

[Lastdog]

Do you Remember  how much you turned up the reg?

[Wayne52]

James after adjusting them I'd say don't open more than a full turn to begin with because that will bring up your regulated pressure considerably.  I did
my 22 one full turn when I put the orings in the airtube and regulator before I even put air in it.  The regulator had 13.8 written on it in sharpy.  I think
my 25 had 14.9 written on it and to be honest I can't remember what was written on my 177.  It does take a lot of air to recalibrate the regulator, that
first 2K that you put in that airtube is a lot of air.  I think these are 300cc air tubes on these guns.

[triggertreat]

Nice string and power Jason!  This puts the DAR in some good light.  I see 29 shots within a 2.06% ES averaging 44.88 FPE.

[nervoustrigger]

Regarding tuning, I suppose I have some good news for you guys because in most cases all you really have to do is adjust the hammer spring to find the knee of the velocity curve.  By doing this simple adjustment, it accomplishes several things:
1.       Efficient use of air / more shots.  Decent efficiency…not stellar.  Can be improved further with an SSG or similar approach that prevents hammer bounce.
2.       Quieter report.  This improvement follows from #1.  Because the OEM hammer spring is too strong, it wastes a lot of air.  After adjusting, it uses less air, therefore quieter.
3.       Well behaved rolloff once it “falls off the regulator.  This characteristic provides a few additional shots after the pressure in the reservoir falls below the regulator’s setpoint.
 
To do this—to adjust to the knee of the velocity curve—all you have to do is gradually increase the hammer spring preload until the velocity no longer increases.  Then back off until the velocity falls to about 97% of that maximum.  Let’s say your max was 900fps, just back off until it falls to 900 * 0.97 = 873.
 
Adjusting preload on the DAR is easy.  Just remove the action from the stock (single bolt on the underside of the forestock with a M5 hex socket).  On the back part of the receiver, you’ll see a disc that’s roughly ¾” in diameter with two dimples in it.  Use a spanner wrench or just a small pair of needle nose pliers to turn it.  Tightening it puts more preload on the hammer spring, thus will increase the velocity.  Backing it out reduces preload and velocity.
 
Be advised the hammer spring on my .25 was too strong even at minimum preload (disc flush with back of receiver) so I had to first shorten it in order to have some adjustment range.  I don’t know if that’s true for the other two calibers but it probably is.  So if you find you are always getting the same velocity no matter how much preload you give, take out the spring and remove half a coil at a time and retry.  All you have to do to access the spring is remove the disc and there it is.  Spring steel is hard…diagonal cutters usually won’t do it but an emery wheel in a Dremel works great.
 
Once you’re happy with it, you want to do something to keep the disc from backing off on its own.  Some threadlocker or a little dab of hot glue will do the trick.  Just be advised that threadlocker cures slowly on anodized aluminum (considered an “inactive” metal).
 
That’s it!
 
From this point, if you are not where you want to be in terms of shot count or energy, it’s time to look at uptuning or downtuning.  Maybe in a subsequent post…

[nervoustrigger]

Don, this evening I’ll link up a copy of the spreadsheet.
 
Triggertreat, thanks for the encouraging words.  The DAR and I had a rough go of it at first…but tight groups healed up those hard feelings   It was a pleasant surprise seeing most of the string falling within 1%.  That makes me wish I had done a couple of strings in stock form to see how much the consistency improved after the rebuild.

[Wolverineshooter]

Jason, this is great and timely! I do have two questions.

1. When you say "gradually increase the hammer spring preload until the velocity no longer increases", do you mean the maximum speed corresponding to each pre-load will no longer increase? So you run a string of shots at one pre-load and find the maximum speed, then increase the pre-load and refill the air, and run another string and find the maximum speed to see if it has increased or not.

2. You mention the spring is too strong to begin with. If you increase the setpoint for the regulator, will the strong spring be a better match?

[Lastdog]

Thanks guys, will start with 22 while I wait for my 25 to come back. Already got the reg turned up one full turn on the 22. Had 13.9 on it. Will work on the spring next. Did you end up with more that one turn?
The add on SG says 250cc.

[nervoustrigger]

1.       Each time you turn the dial to try a new preload, you only need one or two shots to check the velocity.  That’s a good question because on an unregulated setup, you have to run a full string to see its effect on the bell curve.  But on a regulated setup, that is unnecessary.  Just run enough shots to establish confidence.  For me that’s usually two and if they are pretty close, that’s enough.
2.       Yes, if you raise the regulator’s setpoint, more hammer strike (a stronger spring and/or more preload) is necessary to open the valve.  It’s just so strong to begin with that you may find it still has to be shortened.

[Wolverineshooter]

Thanks Jason.

1. I forgot of the regulator for DARs as I am still working on the PP700.
2. I will try to get Wayne's kind of power around 60fpe, but your kind of consistency as I would like to shoot >30gr pellets/bullets at 950fps. How much did you short the spring?

[nervoustrigger]

Lastdog, on shortening the spring, I ended up finding a replacement spring in my assortment that was almost identical except 1.35” long instead of 1.75” long (0.4” shorter).  The wire was slightly larger at 0.062” rather than 0.059” so presumably a bit higher spring rate, but yeah, shortening the OEM spring, I suspect you may end up having to take off quite a bit.  The old “take off half a coil and retry” recommendation is just a conservative way to account for unknowns so we don’t end up shortening it too much.  Fortunately the spring is super easy to get to so it only take a few minutes to iterate.
 
Regarding the reservoir volume, the product description says 250cc but it’s a good bit smaller once accounting for the reg and plenum.  I didn’t think to measure it while I had mine apart but laying a tape measure to the outside of the tube and estimating about where the regulator ends, I think we have about 15” of length with a 1” ID which works out to about 11.8ci or 193cc.

[Lastdog]

Jason, trying to tune my 22 to get your type string. I now understand what your saying about the spring. Do your ember how much you turned up the reg?

Wayne, when I get my 25 back I will be trying to Germany’s a power coyote string like you. Do you remember how much you turned up your reg. And did you then work on hammer or did you get the power some other way?

[nervoustrigger]

My reg is still at its original setting.  It was marked 14.6 or 1.46 which I presumed to represent 146 bar (2100psi).  Based on the pressure at which I began to see a gradual decline in velocity (around 1700psi), that may be right but I suspect the setpoint is actually a bit lower.

[Lastdog]

Ok, Jason, the string you got was by adjusting the your new hammer spring only? The way you explained to me earlier?
So I adjust ( cut if I need to) till I get max speed then turn out till I get a 3% reduction?

[nervoustrigger]

Right!  I did not change the regulator setpoint or the porting.  The only was manipulating the amount of hammer strike.

I think you understand the adjustment but if I try I can interpret this portion wrong:

So I adjust (cut if I need to) till I get max speed...

So just to be on the safe side, let me say it like this.  I'm expecting you will start out at max speed and be unable to get anything but max speed no matter how far you back out the disc.  So technically you will cut if you need to until you can get something other than max speed.  At that point there should be plenty of adjustment to dial in some preload until you get back up to max speed, then back off to 97%.

[Gertrude]

Jason,
I have some ideas for increasing the power that I'd like your opinion on. I feel the plenum is quite small for the .25 cal.

First, what are your thoughts on using a small Ball grinder in a Dremel to cut a radius in the lower forward wall of the valves exhaust port ?

Second, what are your thoughts on increasing the size of the plenum ?
I realize we can't really lengthen the plenum much, due to the location of the vent hole in the air tube for the regulator... So my thoughts are to either cut some of the side walls of the existing plenum making sure to maintain either 3 or 4 "legs" to provide enough strength to prevent the plenum from crushing/collapsing,...
Or
Just drilling as many holes through the sides of the plenum as possible and still maintain the structural integrity of the plenum.

What do you think ?

[nervoustrigger]

The plenum is indeed quite small for a .25 cal.

The spacer is what, about 2" long with a wall thickness of roughly 1/8"?  That means its full volume is roughly 11.26cc.  So perhaps optimistically if we could eliminate half of its material, there would be a gain of 5 or 6cc.

The overall plenum volume (which includes the inside of the valve) is in the ballpark of 17 - 18cc.  Just doing some rough calculations based on memory.

So for a 60fpe tune, that hypothetical total is still rather shy of the 0.5cc/fpe guideline for in-tube regulators.  However it does represent an increase of about 30% which is significant. 

That assumes the skeletonized spacer won't do this:


When the tube is pressurized fully, it has about 2300lbs leaning on it.  Good chance it's going to get Belushi'd so I would want to have a backup on hand before trying it.  A steel spacer would probably be the way to go.

[Wayne52]

Titanium tube with half the thickness would probably work fine but you'd have to find tubing like that. Same O.D.

[nervoustrigger]

Don, here's a copy of the spreadsheet...

[Lastdog]

Thanks Jason. Hope to start on the 22 today. I think I. Understand now.