QB78/79 HPA - Lightest Hammer Strike Possible? - Detune

[sr1sws]

Once I'm done tinkering with my QB79 to pretty much max out performance (and efficiency??), I will want to detune it for longer shot strings.

If I understand correctly, probably the best things to tweak are hammer strike and transfer port size.

I plan on making an RVA, but the question I have is "what is the lightest possible hammer strike?"  Is it zero pre-load or can you go 'negative' pre-load where the spring has some slack before cocking?  I have already flat-sided the striker, probably could take some more metal off - it currently weighs in at 42 grams.

To strangle the transfer port, I'm considering making a small transfer port bushing - possibly fitting inside 1/4" poly tubing.  Bushing would be drilled size 'x' - like you can tune Challengers, Discos, etc.

Thoughts or other suggestions?

Steve

[nervoustrigger]

I have a .177 set up with a lightened striker and slack in the spring before it is cocked.  Supposedly that is a good way to avoid hammer bounce for best efficiency.

[sr1sws]

I have a .177 set up with a lightened striker and slack in the spring before it is cocked.  Supposedly that is a good way to avoid hammer bounce for best efficiency.

Great, thanks!

Do you know what fps/fpe you're generating?

I'd love to shoot wadcutters, but have never found one that works in any of my guns - possibly because the velocity is too high.  I think I read that wadcutters like it between 500-600 fps.  Might be a challenge to get the QB that low, it's currently running about 830fps.

Steve

[nervoustrigger]

It is set up for 15 - 16fpe with 8 - 10gr domes using a 1000psi regulator setpoint.  It does not have an RVA and would probably benefit from one, but it is such a laser and still gets over 100 shots per fill so I decided I will wait until something needs servicing before I fool with it.

Somehow it shoots the RWS Basics (7.0gr IIRC) decently out to about 25yds, despite them going well over 900fps.  By decently I mean about 1/2", enough to ruin the day of many a house sparrow.

[BigTinBoat]

I have a .177 set up with a lightened striker and slack in the spring before it is cocked.  Supposedly that is a good way to avoid hammer bounce for best efficiency.

Great, thanks!

Do you know what fps/fpe you're generating?

I'd love to shoot wadcutters, but have never found one that works in any of my guns - possibly because the velocity is too high.  I think I read that wadcutters like it between 500-600 fps.  Might be a challenge to get the QB that low, it's currently running about 830fps.

Steve

Make yourself an RVA and you will have a LARGE range of velocities to pick from. I built my 78 with a reverse bottle and put the regulator at 1350psi.

Here is my text from when I was testing it.....


Set everything up and got my Winny 9.8g domes.

Shot the gun as I had it with just a tiny bit of preload - 222FPS

Turned the adjuster in 1 turn - 382FPS

+1 Turn - 564FPS

+1 Turn - 688FPS

+1 Turn - 807FPS

+1 Turn - 912FPS

+1 Turn - 939FPS (getting loud and can hear the wasted air)

+1 Turn - 950FPS (getting even louder)

Decide to throw in a 7.0 RWS and see what happens


Here is a link to my build
http://www.gatewaytoairguns.org/GTA/index.php?topic=81046.0

[nervoustrigger]

Steve, one thing worth mentioning is that while you can indeed detune quite a bit with an RVA, the ES tends to climb at lower energy levels.  Intuitively it's because there is barely enough energy to crack open the valve, introducing more variation from shot to shot.  Fortunately if you are downtuning for short range plinking, it probably won't be a concern.  Just saying if you tune the rifle to sling 10.3gr JSBs at 900fps, don't expect single-digit ES for shooting 10m with wadcutters at 500fps.

[sr1sws]

Steve, one thing worth mentioning is that while you can indeed detune quite a bit with an RVA, the ES tends to climb at lower energy levels.  Intuitively it's because there is barely enough energy to crack open the valve, introducing more variation from shot to shot.  Fortunately if you are downtuning for short range plinking, it probably won't be a concern.  Just saying if you tune the rifle to sling 10.3gr JSBs at 900fps, don't expect single-digit ES for shooting 10m with wadcutters at 500fps.

Good info, thanks.  I expect to have to 'strangle' the transfer port as well.  Could be interesting.

Steve

[sr1sws]

I have a .177 set up with a lightened striker and slack in the spring before it is cocked.  Supposedly that is a good way to avoid hammer bounce for best efficiency.

Great, thanks!

Do you know what fps/fpe you're generating?

I'd love to shoot wadcutters, but have never found one that works in any of my guns - possibly because the velocity is too high.  I think I read that wadcutters like it between 500-600 fps.  Might be a challenge to get the QB that low, it's currently running about 830fps.

Steve

Make yourself an RVA and you will have a LARGE range of velocities to pick from. I built my 78 with a reverse bottle and put the regulator at 1350psi.

Here is my text from when I was testing it.....


Set everything up and got my Winny 9.8g domes.

Shot the gun as I had it with just a tiny bit of preload - 222FPS

Turned the adjuster in 1 turn - 382FPS

+1 Turn - 564FPS

+1 Turn - 688FPS

+1 Turn - 807FPS

+1 Turn - 912FPS

+1 Turn - 939FPS (getting loud and can hear the wasted air)

+1 Turn - 950FPS (getting even louder)

Decide to throw in a 7.0 RWS and see what happens


Here is a link to my build
http://www.gatewaytoairguns.org/GTA/index.php?topic=81046.0

Thanks!  I'll check it out!
Steve