Happy Saturday, everyone! I recently purchased an FX Crown (my first PCP), and decided I wanted to try out the JSB MKII heavies to see if I could get better long range results (100+ yards) with the higher BC over the standard 25 grain JSBs. It came tuned out of the box with a regulator pressure of ~122 psi (per the rifle's pressure gauge), and I started working that pressure upwards in an attempt to get the MKIIs closer to the mid-800 fps range, from ~780 fps stock. below are my chrono numbers for 5-10 shot strings, as measured by a Magnetospeed V3 on high sensitivity.
130 psi - Avg: 811 S-D 20
135 psi - Avg: 817 S-D 5.6
140 psi - Avg: 788 S-D 1.0
Without touching the hammer spring adjustment wheel (it remained on MAX for all testing), it is clear that my rifle liked the higher regulator pressure from a consistency standpoint. My only question is this: why would I see a decrease in velocity by increasing from 135 psi to 140 psi? This drop was repeatable over several attempts at testing the different regulator pressures. Do I need to take the rifle out of its stock and increase the hammer spring tension? I have not yet found a good tutorial showing that process, including which direction to turn which screw, and how far... Ted from Ted's Holdover mentioned he went until he felt an increased tension on the screw, but is that the method preferred by most? It was much more accurate at 50 yards at the higher regulator pressure, but I would much prefer a higher muzzle velocity to help out with the top end of the rifle's range.
Thank you very much for your advice, I'm very happy with my rifle so far, and I would love to be able to tweak the most out of its performance while also making sure that I know what I'm doing.
130 psi - Avg: 811 S-D 20
135 psi - Avg: 817 S-D 5.6
140 psi - Avg: 788 S-D 1.0
Without touching the hammer spring adjustment wheel (it remained on MAX for all testing), it is clear that my rifle liked the higher regulator pressure from a consistency standpoint. My only question is this: why would I see a decrease in velocity by increasing from 135 psi to 140 psi? This drop was repeatable over several attempts at testing the different regulator pressures. Do I need to take the rifle out of its stock and increase the hammer spring tension? I have not yet found a good tutorial showing that process, including which direction to turn which screw, and how far... Ted from Ted's Holdover mentioned he went until he felt an increased tension on the screw, but is that the method preferred by most? It was much more accurate at 50 yards at the higher regulator pressure, but I would much prefer a higher muzzle velocity to help out with the top end of the rifle's range.
Thank you very much for your advice, I'm very happy with my rifle so far, and I would love to be able to tweak the most out of its performance while also making sure that I know what I'm doing.
What you must do is adjust the HST screw a half turn at a time (clockwise) and take about 3 to 4 shots over your chronograph and note the velocity. Repeat this until you reach you desired fps. If you notice that the pellet speed / velocity / fps starts to discontinue to increase or actually decreases, back off (counterclockwise) until your velocity reaches its highest fps. Leave it there, thats your true max setting. Its tedious and annoying but thats what it takes to max out your FX Crown. Put your gun back in the stock and then just use the external HST knob to tune for pellet speed. You may find that “4” works for JSB 25.39’s. and “5” is good for JSB 33.95’s. 140 bar is my Crown’s regulator tune and it is perfect for me!
