Need help modding hatsan bullboss

[Hartlycooter]

Hey guys.  I just recently purchased a hatsan bullboss .25 from a buddy of mine.  He was shooting jsb king 25.39 pellets at 862 with it and it shot good.  But im not one to leave things alone for long.  I purchased some fx hybrids along with some other slugs to try through it and the fx hybrids shoot pretty good.  I adjusted the hammer spring a little but didnt have a chrono available at the time so ended up only turning it out 1 turn from where it was.  I want to do the mods to the valve but having a hard time finding out exactly which other parts need to be drilled out.  Im not looking to fully mod it just would like to get the fx slugs up around 900 to 930 fps.  Any guidance you guys can give me is much appreciated

[Rallyshark]

This post will help a lot.  It is on an AT44, but it uses the same valve and the same tuning principals should apply. 
https://www.gatewaytoairguns.org/GTA/index.php?topic=33903.20

Also, this more recent one may help too.
https://www.gatewaytoairguns.org/GTA/index.php?topic=167799.0


I hope that helps

[Back_Roads]

 Try another turn to a turn and 1/2 out yet, I have mine shooting 38 gr slugs 910 fps for a full mag.

[VaporTrail]

I had a .25 Boss at one point in time. All I did was turn the hammer spring as tight as it would go while still being able to cock the rifle.

Shooting the 28gr Benji Domes was around 900 something FPS, if I recall correctly.

Try starting with turning that hammer spring, and if that don't work, try out those two threads.

[kbstingwing]

I had a .25 Boss at one point in time. All I did was turn the hammer spring as tight as it would go while still being able to cock the rifle.

Shooting the 28gr Benji Domes was around 900 something FPS, if I recall correctly.

Try starting with turning that hammer spring, and if that don't work, try out those two threads.

+1,

 that's what I did, mine loves Benji domes....

[Hartlycooter]

It seemed to be shooting slower the more i turned out.  Shots got lower and lower.  I have seen some other people with the bullboss have had the same thing happen.  I will check out the links and see where i get from there. Thanks fellas

[VaporTrail]

Just thought I'd add a little info. I let my stock .22 Bullboss out to play today. Previously I had it set for plinking, just about 100 shots from full fill down to 100BAR. Then I adjusted HS, and it was shooting 21gr Cudas at 672. Wanted more FPE, so i adjusted HS 1 & 1/2 turns CCW from that setting. New speed: 954FPS. 

I'm still not maxed out on HS adjustment, so it could possibly be that you're at the point of diminishing returns with regards to HS adjustment. I'd recommend logging your FPS results from loose to fully tight HS...that should give you an idea of where the sweet spot is for adjustment.

Your results may vary, but if I recall correctly, it was something like past 8 or 9 full turns on the HS where velocity actually started to get worse.

Note your shot curves as well.

[Back_Roads]

 Yes as soon as I see no more gains from HS adjustment I begin to back it off a little at a time until I get the best results, from stock gun parts. Beyond that send it to a tuner