anyone ever make a DIY HDD for a QB78?

[pharoah007]

just wondering.

[bitterman]

Never seen anyone make one.. but,..... the right balance of hammer weight and spring tension on the hammer and you can eliminate most if not all hammer bounce.

Bruce

[AirGunEric]

If you look at one of the units being sold- it only appears to be cap for the hammer pin- likely with a rubber grommet inside. That being said, any type of rubber grommet that has a protective costing to reduce wear should do the same thing. I do have to say that some of the claims I have seen for this type of device ('debouncer')- i.e. double the shot count, no reduction in power, would appear to be a bit optimistic. I think the "bstaley mod" that Bob posted up is likely a better solution- allows for the specific output to be tuned and requires nothing other than some o-rings and maybe some glue to hold them together so they can move/reposition themselves.

[Gerard]

I'm forgetting the guy's name now, but one of the gurus of airgun invention came up with a U-shaped hammer debouncer for the QB years ago and Archer Airguns sells it with a kit:
http://www.archerairguns.com/QB78-HDD-p/qb78hdd.htm
Being a DIY-oriented sort of guy, I spent a lot more of my time than I should have in trying to duplicate his part. Took me a few hours - and I charge $50/hour for my time when working with musical instruments, so I guess you could say it cost me 5 times as much as Archer sells them for. But it worked out well and I don't mind as I love making things work and learning from the experience. Here's a few pictures of what I came up with, made from some O1 tool steel, a lot of filing and bending and fussing to get it right. Works very well. I've also added a single O-ring between the hammer and the steel valve retaining block (mine's a heavily modified HPA QB78), the ring being 0.060" thick, leaving about 0.045" of the valve stem proud of the O-ring. That evened out velocity a bit and quieted the action a lot, while reducing wasted air. Anyway, here's the pictures to revive this old thread. Hope this helps.

[AirGunEric]

Did you happen to have any "before" and "after" pellet velocities?

[Gerard]

Not really, as I didn't install the debouncing device to change velocity but rather to reduce air wasted per shot. Higher shot count being the goal. I was working on the tune to get it down to just below 20fpe, as the gun came to me shooting just a few shots at about 28fpe and I don't need it anywhere nearly that powerful. Or loud. So I chopped off some hammer spring and added about a 6mm long brass preload shim. Took about 30% off the hammer itself with a grinding burr in a Dremel, carving away behind the head and cutting off the sides and bottom as well, leaving about 1/4 of the circumference in contact plus two small corners about 1mm wide each. Lower friction, more consistency. Lighter hammers seem to work for me in modding my previous PCP carbines, a Brocock Atomic and a 2240, so no reason it wouldn't work out the way I wanted with the QB78. I'm very happy with the tune, but await a Ninja 13ci bottle connector from my friend Rick, a machinist, as I want to put the bottle below the cylinder not in front of it. This is to be a backpack-able take down rifle.

Anyway, at a guess I'd question whether a debouncer would have much effect on velocity. More likely it helps a bit with consistency of air use from shot to shot. As does the 'bstaley mod' - adding O-rings as needed to tune hammer depth of impact on the valve stem. Oh, but my target velocity I should state as being about 700fps with JSB Exact Heavy 18.13gr, or 775fps with JSB 14.39gr, either way being just barely legal per the HFT 20fpe limit.