Measuring pressure "sweet spot"

[DKG]

I know you can find your sweet spot/power curve on an unregulated gun with a chrono, but how does one know the pressure? I am currently trying to find the sweet spot on my maximus.

[Ribbonstone]

Can rely on the gauge...even if it's a little off. Will just put a little grease pencil mark on the cover for the start and ending readings. In this use, don't care what the actual PSI is, just that the best shooting lies between the two grease pencil marks.

With rifles that have no gauge, can reconnect the pump and slowly start pumping.  You'll see the pump gauge move quickly, then kind of stop (and often a little "click" as the fill nipple opens).  That would be very close to the actual pressure left in the rifle.

[Back_Roads]

 One needs to shoot  a full string over a crony, then look at the fps readings to determine over what pressure ranges your guns set points perform best at, by noting fill pressure while shooting the crono string.
 Hope this helps some.

[DKG]

Will the weight of the pellet affect the sweet spot?

[Ribbonstone]

In general...no...but some times get a little bit of a change.  Then again, it's hard to fill a typical rifle gauge to within 50PSI (or even 100 PSI), so there is some randomess,but centered on the sweet spot.

[Earl]

I tape a chart to each gun's stock with the gun's favorite pellet plus the elevation and wind correction at 5 yd intervals plus the pressure range and the number of shots.
This is the chart for my Maximus:

JSB 8.44 MV=888         
Dist   Elev   Wind   
yd   inch   5mphVel
0   -1.9   0.0   888
5   -1.2   0.0   856
10   -0.7   0.1   827
15   -0.3   0.2   799
20   0.0   0.4   774
25   0.1   0.7   749
30   0.1   1.0   726
35   -0.1   1.4   704
40   -0.5   1.8   682
45   -1.0   2.2   662
50   -1.8   2.8   642
55   -2.7   3.4   623
60   -3.9   4.1   604
65   -5.3   4.8   586
70   -7.0   5.6   568
75   -9.0   6.5   551
  1750 to 1100         
    35 shots         

[BSJ]

You can use the gauge on your tank.

Fill the gun with the tank. Close the valve on the tank and leave the gun connected. Don't open the bleed screw.

The gauge will tell you the reservoir pressure as you shoot each shot.

See the this video. (Important part starts at 4:26)

And or

 

[DKG]

Thanks everyone! I am currently waiting for my next paycheck so I can buy my new chrono, looking at the Caldwell (but open to suggestions as long as it's not horribly expensive) so I can find the start/end of it. I noticed where it starts to settle down but without a chrono, I am not able to find exactly where. Should I shoot it to empty to find the spread or down to 1k psi?

[Ribbonstone]

Think I posted something like this (in one form or another) about 10 years ago...but it'sw worth a revision.

Can at least "ball park guess" the sweet spot without a chronograph.

But you'd need:

1. A decently accurate rifle so you could tell the center of each group reasonbly well.
2. A rifle that doesn't tend to wander it's groups here-n-ther but stays stable to point of aim.
3. The idea that the slower it shoots, the lower it will hit the target...which is almost always ture. Are some exceptions with stressed airguns (where the barrel is kind of "tweeked" from some odd contact or the air tube is "mucked up" in it's connection to the stock).


So anyway...if you want to give it a try.

Use your best pellets, stable rest, one range, and DON'T TOUCH the scopeadjustments during the process (as it really makes no difference WHERE the group forms, other than it's relationship to the groups that follow).


Just 20 yards (which makes small changes much more meaningful) and a set of targets with a very small volume non-regulated PCP.

(If it were regulated...wouldn't it all, from top fill to regulator output pressure,be the "sweet spot"?)



Which isn't too informational to most folks...but those little 1/4" shifts do mean somthing.

So lets cut the targets out ande kind of bump them up against each otherand draw a line between the group centers:


OK..we're getting a kind of flat bell curve.  COULD change the photo's ratio, making it taller and skinnier to see it in more detail, but it's pretty good the way it is.

Best shots on this rifle are #6-21 BY THE GRAPH.


BUT i DO HAVE A CHRONOGRAPH, and did crhornograph all of thoose shots.

So lets see if the Chronograph readings match my "GUESSTIMATE" from the targets.
(Although I admit to just abandoning the last 5 shots as being "stupid slow").



Yes..it kind of did. Shots #6 to #21 (only 15 shots..butit's a tiny air volume) were prettu consient.From 719 to 740fps is just under 3% velocity spread...and those groups stayed in the most consistewnt POI on the target.

Up to you...you can figure out where the berst of the best lives inside a non-regulated fill. Will take a LOT more shooting for a large air volume non-regulated rifle (maybe 10-14 sets of 5 shot tragets) or it might be evident after only a few targets (for one that declines in speed really quickly).


But the evidence (from many rifles) is that you can get a good idea of where the "sweet spot"lives without a chronograph.

BUT...you won't know just how fast.how much energy eash shot has without a chrongraph...so BUY ONE.

Not convieced the cheap/simple ones and the expensive ones don't all run on the same basic processor...the little chip inside is very likely to be the same, just the "bells and whistles" differ. I don't mind noting down the velocity by hand, so tend tothe cheap ones.

Of the cheap ones, haven't found any real head to head difference between the bottom of the line Chroy and the Caldwell.  One may read 1/2% slower of faster than the other,but that'ssuch a little differnce I can't swear if it's the chronograph or the mood of the rifle I'm shooting.

[Earl]

If you only have one gun to test, try borrowing a chronograph.
You will only need it for 1 weekend.
If you shoot with an experienced shooter it will help.
Shoot from 2000 psi down to where the velocities get 100 fps below the maximum.
Use the gauge on your Maximus to determine the "sweet spot" pressures.
Below are 2 runs of shots from my Maximus from 1750 to 1100 psi.
My old Caldwell chronograph gives mixed velocities.

898 1750 psi
929
903
899
920
888
915
936
924
898
894
902
871
901
897
917
897
898
892
905
915
879
893
869
865
868
892
860
954
862
873 1100 psi


893  1750
895
891
896
877
869
852
915
878
890
945
902
919
917
924
926
910
923
897
928
918
909
914
897
896
889
880
881
876
878
900
873
862
864 1100 psi

[DKG]

If you only have one gun to test, try borrowing a chronograph.
You will only need it for 1 weekend.
If you shoot with an experienced shooter it will help.
Shoot from 2000 psi down to where the velocities get 100 fps below the maximum.
Use the gauge on your Maximus to determine the "sweet spot" pressures.
Below are 2 runs of shots from my Maximus from 1750 to 1100 psi.
My old Caldwell chronograph gives mixed velocities.

898 1750 psi
929
903
899
920
888
915
936
924
898
894
902
871
901
897
917
897
898
892
905
915
879
893
869
865
868
892
860
954
862
873 1100 psi


893  1750
895
891
896
877
869
852
915
878
890
945
902
919
917
924
926
910
923
897
928
918
909
914
897
896
889
880
881
876
878
900
873
862
864 1100 psi

I've been checking with friends, but so far the only friend I have with a (non paintball specific) chrono lives 12 hours away. I'll find one eventually I'm sure.

[Earl]

Try checking a local shooting range or sporting goods store for someone that is willing to help you out.
The same chronograph works for firearms, pellets, BBs, paintball and even arrows.
You just need to shoot from 2000 down to 800 psi.  This will be a string of about 60 shots.
Then you need to run a check string about 100 psi each side of the "sweet spot".
Write down the shot number plus the velocity plus the psi.
You should start at 2000 psi, write down where the gun gauge passes a place where the gun gauge is marked. 
That will probably be 2000, 1750, 1500, 1250, 1000.
Keep the written record of both shot strings.
You may have an area where there is very little change in velocity.
For example 1600 to 1200 psi may only have a difference of 30 fps but only 20 shots.
1800 to 1000 psi may give you 40 shots with a difference of 80 fps.
You can choose what psi to start with and where to stop to refill with air.
I install scopes, sight in guns, find a favorite pellet for a gun, chronograph and make shooting charts for friends for free.
If you were close to Lexington, KY, I would help you set up your gun. 
Use your guns gauge for the psi. 
It does not matter if the gun's gauge is exactly accurate. 
You just need a starting and stopping place for the "sweet spot".

[DKG]

I appreciate that Earl. Thank you for the input