QB78 Hammer Mods

[remtom1200]

There is something missing....a mod I feel has to be done to my QB78 pcp converted spike driver. The hammer bounce has annoyed me since I turned up the heat on this rig, and hearing wasted air exit the barrel after the pellet has left is saddening. My AT44 has such a nice crisp snap to it which is hard for me to figure out what I could do different to the QB as I dont remember any debounce system in that setup on that hatsan. Now I call it the hammer stop, which is the steel dough-nut the hammer slams into after driving the valve pin foward, The AT44 has a brass dough-nut? This is just an idea with no evidence of it making a difference on the QB but if you tap two steel ball peen hammers together face to face they bounce with little effort or force. Tap a pall peen hammer and a brass hammer together?.... you get bored and go mix another drink . Does machining a brass hammer stop sound like an experiment to try in the QB to anyone or has this already been done?

[rsterne]

I think you will find that unless you REALLY have the hammer spring tension dialed up, the hammer in a QB isn't hitting the valve retaining collar (that you are calling a hammer stop).... The hammer bounce is a combination of the balance of the two springs and the air pressure, and the hammer weight.... Have you tried dialing back the hammer spring preload while shooting through a Chrony?.... It is quite possible you have too much hammer strike, and reducing it may well increase your shot count, reduce the hammer bounce, while not losing any velocity at all.... This is assuming you are running a regulated setup.... What pressure are you running?....

Bob

[remtom1200]

I am running an unregulated setup, with valve mods and a velocity adjuster. Im using 1200psi to propel 23 full power shots with jsb exact 18gr(3000-1800psi) which is good for me. The bounce wasnt an issue until I started shooting the AT44 which revealed the QB's waste air out the muzzle. My curiousity is what makes the AT44 not bounce. I havnt changed out the transfer port for a smaller one yet on the QB.

[rsterne]

You're doing something wayyyyyyyyy different than I have ever done with a QB.... I've never used over 2000 psi.... so you must be using quite a bit stronger hammer spring to allow those pressures.... You say "full power" shots.... what is the velocity string looking like?.... What is the volume of your air tube?....

Bob

[remtom1200]

I have the 3000psi pcp conversion tube from Eric, I do not have a chrono at my disposal so I spend alot more time with poa and poi with a given number of shots to determine "full power" to drop off, as well as using chairgun. This has enabled me to tune for accuracy the long way around but has proved to be a good method with the tools I had. There is a correction to my type of pellet.....premier hollow points 14.3gr in the QB, the jsb's are what I use in the AT44. Bob, have you converted a QB with one of the kits Eric sells? I kinda keep an eye out for your tunings as I've learned alot.

[rsterne]

Nope, I've never used a CrMoly tube on a QB.... I use a 13CI tank regulated to 1600 psi.... My current .22 cal does 40 shots at 35 FPE (928 fps with JSB 18.1 gr).... but if I dial it down to 900 fps I get 60 shots....

Bob

[remtom1200]

you know I have thought about that, using my new 50ci ninja tank tethered. I think regulated is good and if I would have liked the looks of the QB79 I would probably bought one of those, might be my next purchase if I can get the 25cal Hatsan out of my mind.

[AirGunEric]

Are you sure it is hammer bounce and not simply far too much air being driven through the valve on the single strike? My only point being that be sure it is hammer bounce that is the issue before trying to find a solution. I might suggest that if you are shooing at 1200psi it is not likely 'bounce' at all, but perhaps too much hammerspring tension. Why not try dialing it down in small steps and see what occurs?

[remtom1200]

I fill to 3000 psi Eric, but use 1200 psi of it. My day off today and im a goin outside to check this air usage. I will be reporting back.

[remtom1200]

So the numbers are in... 3000psi to start, 1200psi end, and 25 shots later at 20 yrds. 0.245 high. I then moved up to 10 yrds and did another 25 shots starting at 3000psi and ending at 1200 psi with a POI of 0.776 lowp_00137.jpg
These results found in chairgun gets me in the 950 - 1000 fps range using my zeroed in range which is 60 yrds. There are variables here I am unsure of to narrow down the velocity