Al, now that you addressed the screws so that you can increase the pressure, you have decreased the safety margin on the threaded portion of the tube.... Until we know what the material is, we have to assume mild steel, so at 2000 psi (using 44K to yield) you have reduced the safety margin to 1.9:1.... That means that every time you fill the tube you are "working" the steel past 50% of it's yield strength, which is into the area where fatigue becomes an issue.... Add to that the "stress riser" caused by the sharp vee at the bottom of the threads, and eventually I would expect the tube to fail at that point.... It might take 50,000 cycles, or it might not, and if the tube is CrMoly you might be just fine.... but exposing the threaded portion of the tube to pressure is the weak point of the design, IMO, once you address the screws....Bob
Personally I would not go above the recomended 1500 psi, thats a risk I prefer not to test while holding it in my hands. Yes C02 can vary all over the board but ive never heard of it going over 12-1300 psi.