HST adjustment
Well, I did the delrin replacement for the brass piece a couple weeks ago. I also did the o-ring mod at the same time. Anyway, I counted the revolutions when I removed the HST adjuster. I thought I put it back right but it seems the head is deeper into the action than before – not sure. This is on a 22 caliber. Well, yesterday I had the opportunity to chrony the gun. When I got the gun, Tony sent chrony results that averaged about 930 with 18 gr jsb. We further adjusted the gun to get it back to 930. Now the head of the HST is pretty far into the action. Is there a possibility that it is screwed in too far but gives the same fps as a looser adjustment? Whereby I’m simply loosing too much air? How far in do others have theirs on a 22? Flush, 1/8 inch, 1/4 inch, 1/2 inch?
How should this initially be adjusted? Should it be simply screwed in until flush with the action, chronied to see where it is and then make say 1/2 turns until it is back up to 900 plus?
All Replies
Screw the HST out about 2 turns from flush.
Start measuring FPS at each 1/2 or 1/4 turn in. Plot the FPS vs screw in position and see if you are going past the point where the relationship begins to flatten out.
If you are past the point that screwing in no longer increases FPS by about the same amount, you are probably getting to the point that air is being wasted.
Back off a bit from that point (if you find such a point in your graph).
That is now your starting point, but still won’t be the HST setting that gives the most consistent FPS as air is used.
Next, do a shot string from full down to about 100 bar. Take note of what pressure you are at occasionally. Maybe every magazine start. This will get you both the FPS and fill pressure relationship.
The curve will be one of three shapes.
“Above the knee” —- A bit more energy delivered per shot, but uses more air. Some change in FPS across useful shot string
The FPS curve is pretty flat but has a downward slope across the regulated range. When it comes off regulator, the curve simply curves down. (no upward climb of regulator)
“On the knee” —- Hammer spring releases just the right amount of air from regulated chamber to maintain consistent FPS throughout useful shot string
The FPS curve is flat during the regulated range. There isn’t a slope to the flat portion, just flat. When coming off regulator the FPS simply drops off.
“Below the knee” — Hammer spring releases less air than the regulated chamber can supply. Lower air usage, but FPS will climb coming off regulator.
The FPS curve is flat during the regulated range, but once off regulator, FPS climbs above the flat portion for about 10-15 shots and then drops off.
On the Knee is where I am keeping my HST balanced with the current regulator setting.
None of the choices are gospel best. Choose your priorities of consistency, shot count, and power and adjust accordingly.
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Thanks! It’ll take me a while to digest this.
Mine is currently about 3/8 inch or sightly more in past flush. Just didn’t seem right.