Quick test of a cheap HPA bottle (800-850psi output)...watching the BBQ and testing airguns.
At least the two tested do shoot down a little over their range of pressure. Its not a whole lot, averaging 12 - 17fps less at 900psi than they do at 3000psi.
Test gun is a pretty simple conversion. Added a QB gas block and an 850psi out put HPA bottle. Has a stock valve stem, bulk valve body, but some fiddling around with the striker spring to get it running its best at a (more or less) constant pressure. Added a gauged on/off to keep track of the regulator out put.
Started at a max fill of 3,000psi. Used unsorted JSB 13.4gr pellets in this test (have proved to be the most velocity consistent pellet tested).
Looked at in groups of 10 shots
1-10 = 674fps average (678-670)
11-20 = 673fps average (676-670)
21-30 = 673fp average (675 669)
31-40 = 672fp average (675 668)
41-50 = 669fp average (671 667)
51-60 = 667fps average (671 666)
61-70 = 667fps average (671 664)
71-80 = 666fps average (670 664)
81-90 = 665fps average (669 663)
91-100 = 664fps average(668 662)
(There may be a few more shots at a reasonable velocity drop, but its pretty well off regulator by shot #95-100, and down 35fps by shot #105.)
Looking at the maximum recorded speed and the minimum, there is a 16fps spread over 100 shots (shot #3 = 678, shot #99 = 662fps).
Looking at the averages of 10 shots, there is a 10fps spread.
Looking at the most variation inside any 10 shots, there is a 7 fps spreadthat does indicate a nicely uniform pellet.
Not the most accurate gun in the locker by any means. Part of that may be the 4X scope, some due to the lightweight, but most of it is due to the standard Crosman barrel.
One fill...20 5-shot groups @ 22yards:
Interesting to note shots #41 to 50 landing a bit high. Even though PCPs are recoilless, I accidently used a harder front bag on those shotsand they landed higher. (Spilled my coffee on the front bag and switched to another (firmer) onenoticed the difference in POI and switched back to the (coffee wet) softer bag.)
Interestingly I get a spread of about 10-14ft per second spread and less accuracy on my .22cal AS2250XT fitted with an 18" CR barrel, 850psi. However interestingly my AS2260XT fitted with 18" .177cal has a 5ft per sec spread and shoots 5mm groups at 20yrds!
I think its because of the higher velocity. Try a softer valve spring, (GMAC do one 30% lighter, same size as standard with more coils) stock hammer spring and bump up the bottle to 1000psi. If you have to play about with the hammer spring, fit a power adjuster to fine tune it, heavy hammer equals heavy trigger though so lighter valve spring is the way to go. I think you will see an improvement in accuracy. If not try re-crowning the barrel
Great job though! Your doing well.