No Chronograph testing (EDIT)

[Ribbonstone]

Have posted this a few times.

You can figure out where the sweet spot is without a chronograph.  Won’t know how fast, or how much energy per shot, but can figure out where the most consistent velocity is in a fill (which is what a sweet spot is).

Need a gun that shoots really well (well enough that a small shift in group center is noticed).

Need a target with rows of identical (or nearly so, I hand drew this one) targets.

Need 25 yards of range (may be able to do with 20yards if it’s a super accurate gun and you can detect tiny group center shifts without blaming it on variations in hold).

Would shoot 5 shot groups if you expect the gun to have a long shot count.  3 shot groups if you expect the sweet spot to be short.  I used a test gun that is sure to have a short shot count, so used 3-shot groups.

The test gun filled to 175BAR and shot down to 95BAR
Target shot at 25 yards, 3 shot groups:


BY EYEBALL: Looks something like from shot #6 or 7 to shot #21 is in the best area.  Call that 15-16 shots.

Now if you are having trouble judging the results, then get a pair of scissors.  Cut the targets small, but still including enough to line them up evenly.


Once cut, line them up and tape/glue them down.  Find the group centers and connect them with an ink line.



WHAT TO DO WITH THIS?

Evidently the first shots are slow, so rather than fill to 175BAR, I fill to 165BAR.  Will take 15-16 shots, then refill the gun back to 165BAR,

NUMBERS:

While I was shooting this, I also shot over a chronograph in order to get a direct comparison.  This is NOT a mega-power gun; it is currently loafing along at about 10 foot-pounds.



But we do get to see just how well the EYEBALL predictions match the chronograph.




By careful target testing and by EYEBALL, came up with 16 shots.
By chronograph, came up with 17 shots.

Close enough.

Do get to know the exact velocity and energy with a chronograph, which is very valuable information, but you can figure out where your gun is running best without one.

EDIT:

PITFALLS:
1. Noise does not always equal power.
Can make a really loud 600fps gun and a really quiet 600fps gun, then hand them off to strangers to shoot and have them guess which one is faster.  They always pick the loud gun.

2. Recoil does not always equal power (mostly for springer shooters).  Just because it twitches, twangs, and jumps all over the place does not mean it is going faster than another gun that is smooth and mild.

3. If you shoot at the same range all the time, you can get a guesstimate of velocity by the time it takes to hear the pellet strike.  It is a very rough estimate, seldom closer than 100-75fps except by luck.

4. Trajectory can help, but realize that some pellets are going to shoot higher at short range than others, which really screws the long range drop numbers.  For a gun with a known scope height, zeroed exactly at one known range, can get an idea of velocity (but because pellet BC is variable between guns, the results are just going to be ball-park).

[rsterne]

I think this can be a very valuable tool, but I would like to add one more Pitfall.... Some PCPs can exhibit a non-standard distribution of POI depending on the pressure in the reservoir at the time.... I observed this first in my Disco, which shot high at first (at 2000 psi), then dropped, then climbed again, and then stayed relatively level for POI even though the velocity was dropping towards the end of the string.... What I "think" was happening is that the reservoir (which is under the barrel) was growing in length and pushing up on the barrel when full, and less so as the pressure dropped.... What I "know" worked was loosening the barrel band setscrew so that the barrel was held vertically, but could slide longitudinally.... I then made a new barrel band with an O-ring instead of a set-screw which accomplished the same thing but in a more repeatable manner (and no worry about the set-screw coming loose).... Once the barrel was unaffected by the pressure in the reservoir, then it seemed to react as per your post.... A fully floated barrel would probably have worked as well....

Bob

[airpuffhunter]

excellent info I don't have a chrono Right now,  I will try that

thank you

[Sqrl Klr]

Bob that solve is fascinating. I've never heard of it.

What pcp's are not prone to POI shifting? I thought they may all have it. 

[rsterne]

All PCPs are prone to the type of POI shifting in the OP.... the trick is to keep the ES within a narrow enough range that it doesn't matter at the range you are shooting.... The somewhat "random" POI change I experienced on the Disco shows a flaw (in the design or my gun I don't know)....

Bob

[jimmie lee]

Ribbonstone, this is basically the method I've been using for my Disco and TalonP for the past few months though I didn't do the cutouts and paste them to show a physical shot string and was able to determine starting fill pressure and ending fill pressure. I use two barrel bands and experience what rstearne mentions in expansion/contraction of the pressurized tube so will see if I can get away with the one band and see if that anamoly dissipates. It will be interesting to see how close I am when I get chrony time.

[Nomadic Pirate]

....

[Ribbonstone]

Something to try. Am sure that are more ways to stabilize a Discovery, but this one has worked great for me.

Will tell you up front it’s easier to do than it looks.

Really got POed at the wandering zero of the Discovery. Shot great, but could not depend on it to shoot to the same POI from one day to the next.

So I looked at how the Crosman Challenger PCP is set up and decided to add a barrel band right ahead of the breech.

1.   Locate where the band should go (and there really is not enough room between the gauge and the trigger unit, so it has to go just ahead of the gauge).


2.   Cut the sight base into two parts at that point.  The band will fit BETWEEN the two halves.  Looks good this way folks are always asking how I got the band to go over the sight base (simple I didn’t, got the base to go on both sides of that band.)
3.   Inlet the stock.  Only has to be nice looking where folks can see it .

4.   Use this barrel band to draw the barrel DOWN tightly.


The forward band ended up with 6 set screws.  A “tripod” of screws that tightly lock that band to the air tube and a tripod of screws that just BARELY let the barrel slide in and out (but no sideways play).



(if you are wondering about the screw’s location, it’s best to drill and tap the THICKEST part of the metal band.)

After playing with various locations for the forward band, didn’t find any real difference in accuracy, but it seemed that putting it at the end might be better at keeping the POI stable from day to day.

[Sqrl Klr]

All PCPs are prone to the type of POI shifting in the OP.... the trick is to keep the ES within a narrow enough range that it doesn't matter at the range you are shooting.... The somewhat "random" POI change I experienced on the Disco shows a flaw (in the design or my gun I don't know)....

Bob

I believe it's likely the design. Shame I didn't know about this issue beforehand as that was the reason I spent over $1k to have a dependable gun that would always shoot on bulls eye. Right now I'm in the same position that I was in with a $40 daisy 880 with pellets shooting over 2" groups at 15yds which is getting rid of my new found pcp addiction I must admit since I will miss a lot of pests this way. That's a lot of money to blow only to be having the same problem as before with cheap guns. Would have been better to buy a rainstorm or something of better quality that shoots great out of the box without any tinkering needed after all the issues I've encountered with the disco over the past 2 months. Lesson learned the hard way.

[Ribbonstone]

That is certainly one view (although random chance has a hand in this as well, and money spend does not have to always equal increased performance).

But for a few hours work and a small amount of $ for a few parts (drill, tap. Screws, band.etc) can have a PCP that challenges a shooter to be as good as the gun. 

Am reminded of a commercial: Learning what makes one tick….priceless.