Hi Sharroff,
Thanks ever so much for the photo. I never knew what this screw did and couldn't find much information about it elsewhere.
But after reading your report, I decided to give it a try on Saturday (and as my wife was away for the weekend, this was a perfect opportunity) and I ended up spending over 6 hours checking and fine tuning the balance between power and accuracy.
I have a .22 Ranchero and use either AirArms Field 16gr or JSB Exacts 15.9gr (my retailer said to try Webley Accupels, as he uses them on his FX rifles, but they were dreadfully inaccurate for me). I have 3x 8 round magazines, shoot only on full power and refill after they have been used up.
AIR ARMS FIELD 16gr
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I first chronographed the original setting to act as a base line in FPS, then plugged the numbers into a spread sheet and line graphed the results. I knew magazine 2 was the most accurate, but didn't really know why until I saw the graph. It turned out that the power curve was erratically increasing power on the 1st 8 shots from 716fps to 744fps. Magazine 2 carried on increasing for the next three shots from 749fps - 754fps and then gently curved down for the remaining shots to 734fps.Magazine 3 plummeted from 731fps - 654fps. it looked like a trail of ants crisscrossing each other.
So I decided not to mess around and wind the screw all the way in and work from there. WOW!!
The power upped to a max 846fps and a near-liner drop of approx 8fps on magazine 1, 12fps on mag 2 and 14 on mag 3.
Mag 1: 846fps to 785fps
Mag 2: 775fps to 691fps
Mag 3: 680fps to 581fps
The graph now showed 3 smoothly decreasing lines for each magazine. Also I recorded the air in the gun after each magazine.
Mag 1: 170 bar
Mag 2: 120 bar
Mag3: 90 bar
I ran out of Air Arms Field and switched to JSB Exacts.
JSB EXACTS 15.9gr
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I repeated the full power shots again and got very similar results.
Mag 1: 838fps to 785fps
Mag 2: 799fps to 692fps
Mag 3: 681fps to 597fps
Mag 1: 170 bar
Mag 2: 120 bar
Mag3: 90 bar
When I tested the accuracy over 22m repeatedly, it showed that Mag 1 was high by about 1cm, Mag 2 was spot on and Mag 3 was low by 0.5cm for the first 4 shots, then dropped between 1.5 to 2cm when the fps was about 630fps.
So I started to wind back the power a quarter of a turn and retest for accuracy and eventually settled at a full turn from tight. this gave me consistent groupings on a 2cm circle. the resulting fps figures are:
Mag 1: 818fps to 785fps
Mag 2: 775fps to 700fps
Mag 3: 684fps to 606fps
Mag 1: 170 bar
Mag 2: 120 bar
Mag3: 95 bar
Now I just need to get some more AirArms Field pellets and recheck their accuracy, but overall really pleased, so thanks for the tip.
By the way I couldn't get anywhere near 900fps and wonder if yours is a .177?
Thanks again.
Regards Peter