This isn't much help but here goes. My gun is not the same tho, I have a Tactical .22 but in 500mm, factory settings and factory pellet liner. So, take this with a grain of salt, but it's what I'm getting. I'm shooting 21 H&N slugs at 810 (yeah, a little slow but they are accurate.) and I'm getting 65 shots. Fill to 230 and refill at 85. The last few shots are between 805 -> 790. So, 60 shots comfortably. If I use 18.3 AEA pellets I get 870 for those 60 shots and still get the last 5 at 860. I guess what I'm saying is that your 600mm barrel and bigger air tube, you should be getting way more shots than 14. I tried putting a gauge in my plenum so that I could read the reg pressure, but I couldn't get my spare mini FX gauge to seat without leaking. I tried Orings and a bonded washer, still leaked, so I gave up and put the plug back in. But, when shooting, I can hear the difference in the shot sound when my gauge hits around 90-ish, that's also when my FPS starts getting lower. Great gun so far, but a little annoying about the plenum gauge issue. If you get your gun figured out, let us know how you tuned it. I'd be curious to see how these tacticals do.
