I wanted to do a little pictorial sequence of a .22 cal QB79 HPA build I have been working on for GTA member UCChris.
Chris wants this gun for 50+ yard hunting duties so the goal was to get over 25fpe and let the shot count fall where it may. The tank block on a 79 almost butts against the front of the valve, meaning the plenum volume is essentially the valve volume. With so little on-deck air to work with, that makes it a little trickier to reach the 30fpe level. But I thought that would be a fun challenge to take on, so off I went.
First up, I knew we would want a fairly high regulator setpoint of 1500psi to achieve the energy goal. So a bit by experience and trial and error, I assembled a Belleville stack onto a Ninja regulator stem and hit a setpoint of 1550psi according to my precision gauge:
Refer to Bob's detailed thread for info on how to do this: http://www.gatewaytoairguns.org/GTA/index.php?topic=49648.msg469843#msg469843
Also, if you use a hand pump like me, a very small "tank" makes the trial-and-error process much less of a chore:http://www.gatewaytoairguns.org/GTA/index.php?topic=73635
With that done, time to make some metal shavings!Rear Velocity Adjuster
Drilled the end cap for a rear velocity adjuster (RVA). It's a 10-32 x 1.5" screw through the end cap which can be threaded in or out to push against the back of the spring guide, whereby adjusting the amount of preload on the hammer spring. Making relatively small adjustments to preload has a profound effect on velocity and air usage (efficiency), so the effort to do this mod is well worth it. Notice it also has a #6-32 set screw intersecting it to lock in the adjustment.Cock-on-open mod
The cock-on-open mod is another worthwhile change. The action is more natural I think. Also, the original cock-on-close action masks certain potential problems like pellets hanging while chambering or fitting too loose or too tight. This mod makes it easier to detect those kinds of issues.
First, milled a rear downslot for the bolt handle. This change allows the bolt to be latched back out of the way to make it easy to load a pellet. At higher power levels necessitating a fair amount of spring preload, the bolt probe may otherwise push forward into the loading tray, making it difficult to insert a pellet.
The other piece of this mod is to remove the rear pin in the spring guide.
Lastly a 0.6" long spacer was added to the shaft of the spring guide. This spacer is simply a section cut from an ink pen. Stacking washers on the spring guide would also work fine. Sorry, I did not take a picture of it.Securing the barrel
This modification helps secure the barrel in the receiver which improves shot-to-shot consistency. I located a spot on the receiver behind the factory set screw, in between where two of the barrel O-rings reside, then drilled and tapped a hole for an #8-32 set screw. An M4 would also be a good choice if you want to stick with metric fasteners. The scuff marks have since been cleaned up and cold blued with Van's.Air flow modifications
Ported the barrel to 0.170", same as the ID of the poly tubing that is used for the transfer port seal. Then radiused the port inside the barrel to prevent a sharp edge from marring the pellets as they are chambered.
Drilled the nose of the bolt probe out to 0.170" as well:
And enlarged the side opening:Valve modifications
Milled 1/4" wide slots in the side of the valve body and drilled out the center hole. Reduced the poppet OD from 0.371" -> 0.340" and polished up the brass. Drilled out the valve throat from 0.194" -> 0.219". Tapered the valve stem from 0.117" -> 0.077" in the region where air flows around it and up through the valve exhaust. Also radiused the inside edges of the exhaust (in the throat) to reduce turbulence. And finally, fitted a conical valve spring from Daniel's (rdsail) group buy, a Century Spring TA-2122.Increasing the plenum
Bored out the center of the tank block to increase the volume of regulated air available to the valve during the shot cycle. I think the diameter was 7/16" or perhaps 15/32", and as deep as I was comfortable drilling so as not to punch through into the holes where the tank block retaining screws go.
Before reassembly, I thoroughly deburred and polished things where appropriate, especially where O-rings might get nicked on installation.Chrony testing
A little twisting of the RVA screw to find the maximum velocity yielded...
23.9fpe with 14.3gr Crosman Premier hollow points:
26.1fpe with JSB 18.13gr heavies:
Goal met! That's running pedal to the floor, so to speak...quite loud and surely using a lot of air. I would want to back off of that slightly for better efficiency.
However, wouldn't it be nice if we could hit 30fpe? As it turns out, there's more energy to be obtained by a rather simple valve modification so that will come next.
Photobucket can die.[/edit]