Pft tuning question on a Prophet?

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igolfat8

igolfat8

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Oct 7, 2018
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So during our recent match I noticed the report of a few competitors rifles had that distinct Pft report that says to me that it’s tuned efficiently. I struggle to get that same tune on my Prophet. I set the reg, run the HS in until it reaches max velocity then back it off until the velocity drops 3% which should be tuning to the knee. But I don’t get that same Pft report. Any suggestions?
 
igolfat8 said:
Sometimes when I shoot down below the reg setting I can hear a report that is approaching what I am describing above. Does that mean I need more hammer at the same reg setting?
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As pressure falls below the regulator setting, valve dwell increases so that is the opposite direction of what is desirable to reduce the report. I guess I don't know the pft of which you speak.
 
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I added a turned down bolt to the back of the hammer for weight and semi hammer spring guide. I was able to turn the hammer spring adjustment all the way out for no preload with the same reg pressure. It is very quiet and others I have shot around with FXs were shocked at how quiet it was. It does have daystate baffles and a 3 section 30mm huma mod on a .22 prophet compact. I know what you mean by quick puff of air. My taipan standard and daystate huntsman sound like that and thwy have short heavy springs. The prophet can do with a slightly heavier hammer so less spring is needed.
 
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Well I achieved the quiet pft report today by backing off the HS gradually until I could hear the report change audibly. Tune is reg at 115 bar and HS 1.5 turns out beyond flush, 865 fps with 18s. Shot count went from 50 (HS flush) to nearly 90 shots with HS out 1.5 turns. Even though air usage was much more efficient and shot count nearly doubled the downside is that the spread and SD numbers went way up. When the HS was flush the SD was 1.5 and when the HS was 1.5 turns out the SD went up to 9.7 with very similar velocity numbers.

I’m open to any ideas, explanations or suggestions?
 
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igolfat8 said:
Even though air usage was much more efficient and shot count nearly doubled the downside is that the spread and SD numbers went way up
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That is the expected result from backing off the hammer spring well below the plateau velocity. What is happening is the gun is operating at a state of partial valve lock, and the velocity is disproportionately affected by minor differences in hammer strike from shot to shot.
 
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I did the 95% of my max velocity. Picked a reg setting and maxed the hammer spring in to see the max velocity achieved. Then backed off the hammer spring till the velocity was 95% of the max velocity. 125 bar got me around 920-930 fps for 18 gr pellets. 95% of that was my target speed of 880 fps. This was all done on a compact .22 performance.
 
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igolfat8 said:
So if I increase the HS, how do I know where to stop so as to keep the air efficiency while lowering SD?
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As with most things, there is a tradeoff. You can get phenomenal efficiency (and a very quiet report) by backing off the hammer spring tension to operate ever closer to valve lock but consistency will suffer. It can work very nicely for suburban pest control out to 30 yards or so, or for indoor shooting. Not so good for longer distances where the velocity spread results in unacceptable dispersion.

Joe's description is the way to achieve the best balance of consistency and efficiency. Gradually increase hammer spring tension until the velocity no longer increases, then back off the HST to 95 - 97% of that maximum.
 
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I tried tuning to the knee by increasing HS tension 1/2 turn at a time as you and Joe described above but I did not see the velocity changing hardly at all even after nearly 2 full turns out or 3 full turns in from flush. The only adjustment that made any real velocity increase was the reg. I wonder if something is broken or not working correctly? I have a full size Prophet LR and the HS makes very noticeable and broad changes in velocity.
 
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ig@8, I think your Reg is too low, so when you’re increasing hs tension, you are over-driving the valve. Try 125 bar again, and see if the hs adjustment has more impact on velocity. My Prophet at 125 bar is sending 18’s @ 960-970 with plenty of headroom. The hammer and spring are very easy to access, so you can get to it to make sure all is well.
 
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You always need to start at the beginning with a gun. After that is established, only then you don’t have to always go back to the beginning with the HS. Leave the gun at 115b or whatever. Back that HS adjuster the whole way out. I’m talking protruding from the buttplate a little. Start shooting over the chronograph. Go a quarter turn at a time until your velocity peaks. Once it peaks and you see no change, and I’m talking the changes you were seeing from the beginning when you were going a quarter turn at a time, stop, you found your plateau. If you subtract let’s say 5% from that speed and it isn’t fast enough, increase your reg setting and turn your HS in until you find your new peak speed. Bust out your calculator and do math again. The reason you must start from the beginning or almost no HS tension is because you need to see how your gun jumps in speed. Guys that don’t go back to the beginning can wind up wondering around in the dark with their hammer spring because of a guns inability to shoot 935, 934, 936, 935fps when not tuned perfectly. Guys start thinking that 940fps they just fired was a velocity increase but all it was is the guns horrible ES. So they keep turning the screw and get lost.
 
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