another look at FPS variation vs POI (group location)

[Ribbonstone]

Just to keep the velocity variation/point of impact discussion going (as it is important):

Retuned the old 2260 Carbine/HiPac. With a major change or two, had no real idea what the pressure it would run it’s best, so started with what I thought was a little over-fill and ran it until I was sure it was well under optimum.



Pretty much the LEAST accurate PCP in the safe but I’ve started each hunting season with that old carbine and intend to start this one with it too.
5 shots each/20 yards: Over the years it has changed. Started off as a 12gr. CO2, then morphed into a bulk fill co2, tanker co2, tanker HPA, and HiPac PCP.



Now if the game was keeping all the shots in the black part of the target (call it 1.2 inches) then all 35 shots would be “good nuff”.

Looking at the target more critically, can see that the POI starts out low (shots #1 to about #11), gets kind of stable (shots #12  to about #28), and then falls back low (shot #29 to #35).

JUST FROM THE TARGET, would predict that the “good shots” would be something like the shots from #10 to about #30.
So I filled it to 2300psi and shot 20 shots (my closet approximation of the 20 best shots in the 1st target):

.

WHAT I’D LIKE TO IMPART: YOU can figure out where the best shots live without a chronograph if you are very picky about POI and have an accurate enough rifle to make 1/8” or ¼” POI shifts meaningful (“meaningful” in that any shift from POI is due to the gun and not the shooter).

NUMBERS TO BACK UP THEORY:

Shot string:

Percentages:


This is just numeric confirmation, I ALREADY KNEW (1) where the “saweet spot” lived in terms of shot count and (2) where it lived in terms of fill pressure by JUST SHOOTING TARGETS.  The chronograph data just (1) gave me an energy rating and (2) confirmed what the target already told me.


BOTTOM LINES:

#1. Even with just one barrel band, there is no real right-left shift in this one-band version of a Dico/2260 conversion.  Is a good bit of up-down variation over 35 shot, much less varation overt the center-core 20 shots, but not all that much right-left shift.

#2. 35 shot target showed that about 20 shots were uniform and good enough to say on a pellet tin lid if I were plinking (even with a +100fps varation).

#3. “Center-core” (or the sweetest of the sweet spot) shots averaged quite a bit better (and more on-target) than the shots with more variation.

#4. PUMP FILLER.  Why would I want to put 49 pumps into a rifle to get 35 shots that varied by ½” (2900 – 900psi) when I could put 28 pumps in for 20 shots (2300-1870psi) that vary by 1/16th inch?

[rsterne]

Great results, as usual.... I just checked it against my latest graph, which shows that for a pellet with a BC of ~0.03 at 700 fps you need about a 3.2% ES to stay within 1/2 MOA (~0.1") at 20 yards.... If the black on your target is ~ 1.2", then each ring must be about 0.1".... and the second set of 4 groups (20 shots starting at 2300 psi) really don't show more than 1 ring of vertical movement at the center of the groups.... I just love it when theory and practice come together....

Bob

[nielsenammo]

Great share. Nice to know the real shooting matched the theory as well.

[Ribbonstone]

Cat skinning has multiple variations...sometimes the empirical rather than the theoretic makes the point better for us visual-learners.

[Oldblades]

sometimes the empirical rather than the theoretic makes the point better for us visual-learners.

You really hit a home run for me on this thread Ribbonstone. 
Thanks for the great post.

[mercury]

Very interesting post and well illustrated, thanks

[Ribbonstone]

Chronograph makes things easier and gives definitive numbers (velocity/energy). But lack of one doesn’t mean you can’t figure out where the sweet spot is.
Even without a chronograph, just shooting targets, can figure out where the most stable run of shots live. Just looking carefully at the targets, I’d have ended up at the same place without the chronograph (I wouldn’t have known the velocity/energy, but would have ended up shooting there).

[SpiralGroove]

Hey Ribbonstone,
Keeping it simple, within basic shooting "rules of thumb" is at least 90% of the accuracy solution.
The other 10% or so is me .............. Unfortunately, I'm not sure there's a solution for that -> on the horizon
Thanks,
Kirk