Bottled QB78

[Monkeydad1969]

Thanks for all who replied.  If I go forward, I will most def ask you all for help.

Joe

[Ribbonstone]

Lets put it this way.

Once you go to an HPA bottle, the pressure to the valve is the SAME from shot to shot.

So there isn’t any real reason to tune for a self-regulating sweet spot like there is with ever decreasing pressures of a PCP. Does pay to figure out a graph of air-use vs. energy to get the gun efficient, but the air pressure to the valve stays the same until the pressure derops under regulator pressure.

(Although there is some evidence that the regulator gets a little spastic BEFORE then).

For power, it gets basic: smooth air flow, streamlined valve,larger air passages, longer dwell.

[nervoustrigger]

I got a QB79 (.177) from AGD shipped for $95 -> God help me with the addiction

My, my.

Weak-willed and motivated to drill a hole in something...anything.

Quite the combination 

But to your question, you certainly don't have to be a hunter to love these rifles.  As Robert aptly shows, you can enjoy a ridiculous number of accurate shots on a fill for punching paper or reactive targets.

One fill of the 850psi .22 QB79 in 5 shot groups (20yards):

Over 200 shots, yikes!  Did you need a nap afterwards? 

[Ribbonstone]

Started out to find was “as many shots at 12 foot pounds” as I could get.  Didn’t think I’d get more than 150.  Ended up shooting 215 times, but the last two groups were slow and low on target.  Believe there is still 10% more to squeeze out.

Kind of pug-ugly little 79:


When looking for more power, will go with the longer 78 tube to get the on-deck pre-regulated air volume.  With the 79, there is only a small volume of air between the reg and the valve, which makes it just as well suited to moderate power shooting, but less well suited to higher power shooting.



THIS IS JUST HOW I SEE IT IN MY HEAD, the numbers are COMPLETELY MADE UP. It’s just my mental construct to keep the processes straight in my head.

How can two guns at the same pressure, same valve, same air volume passing through the valve, end up shooting at different speeds?

Lets say you have two .22 HPA QB’s, a 79 and a 78, running at the same pressure (we’ll make it easy and use a round number like 1200psi).

Will also say that the valve moves 10cc or air per shot.

Lets say the 79 has 20cc of  air between the valve and the regulator and the 78 has 50cc.

And will say that only the air trapped between the valve and the regulator is of any use for the shot (regulator replenishes the air too slowly to add to the pellet’s speed).

Given all that, then the following.

QB 79 starts at 1200psi.  Valve dumps 10cc.  That’s ½ the available “on deck” air so the pressure drops to 600psi by the end of the shot.  So the average push is 800psi.

QB78 starts at 1200psi.  Valve dumps 10cc. That is 1/5th of the available “on deck” air, so it’s pressure drops to 960psi. So the average push is 1080psi.


Could get all aggressive and smack the &^^& out of the valve and dump 15cc.

QB79 would go from 1200 to 300psi, for a net push of 750psi.

QB78 would go from 1200 to 800psi, for a net push of 1000psi.

[Monkeydad1969]

Thanks Ribbonstone.  Now my brain hurts...

[Ribbonstone]

IDea is that to get speed from a small volume (like the 79 HPA) would be best to go with higher pressure.  To get speed from a larger volume (like a 78 HPA) can do it with releasing more air volume.

Basically, when you have a bigger water tank, can run the hose longer without losing as much pressure.

[SpiralGroove]

Hey Jason, rsterne & Ribbonstone,
Really wanted to stop the airgun tuning thing, as it's becoming compulsive for me - I just love tinkering.  Unfortunately, the QB79's price combined with the thought of Bob's Maxi valve (described in his stage 2 mod.) was just too hard to resist.  Besides, the gun is obviously a no brain-er Great Deal aka ....B50/B51.

Ribbonstone, I think you were trying to say the QB78 has a larger plenum and hence is not affected as much by pressure loss, due to the 10cc shot.  I think Bob's QB79 (Maxi Valve) solution was to increase the plenum to 12+cc's, increase the valve throat size and increase tank pressure to 1500/1800 psi.  This mod gave a substantial increase in power, using the smaller QB79 tube size.  Is that about right?

Bob, as much as I tease (its my way -> my bad sense of humor) I'd be lost without your level of insight

Kirk

[Ribbonstone]

Yes, just in simpler terms.

[rsterne]

I tried 1500 psi, gained ~ 3 FPE, but lost over 20 shots.... so I backed it down to 1200 psi for the final tune....

Bob