Flash from the past..
While in a turkey induced daze, I was thinking about the next airgun I ''need''and almost ordered a. 25 break barrel that got my attention, but then it dawned on me that I have a. 22 cal qb 79[do you guys remember the QBs?]collecting dust and a little rust and I want to make it a HPA gun so, has any of you converted one,? and if you have ,mind to share what I need for it? How hard is it to reseal ?Mine is leaking air under the breech . Thank you guys, you are a great bunch. .Now ...back to work on the turkey left overs 😋
Land and Liberty.......
I did mine with a thing called a PowerMax from TalonTunes. It screws into the recepticle for the CO2, add as many 4 inch extensions as you want, and finish it off with a section that ends with a foster fitting. Mine worked great. Don't know if you can still get them. Then I put the QB into a Richard's Microfit laminate stock. Turned out nice.
I don't know, that sounds kinda questionable...taking a CO2 rifle and converting it to HPA.
But hypothetically you could just take out the grub screw to remove the barrel and just screw on a 850psi Ninja bottle. Minimally that's all you have to do to convert a QB79 to a regulated PCP.
However it might be more enjoyable if you hop it up by improving air flow and increasing the pressure. First and foremost, pin the tank block for better safety margin.
A Ninja bottle has a 1.8k burst disk so that will keep the maximum pressure from ever exceeding that of CO2 on a hot day.
If you're hungry for power, you can install a stack of 0.032" Bellevilles and bring the pressure up to as much as 1400 or 1500psi.
Any higher and the burst disk is likely to rupture, particularly if taking it from the cold outdoors to the warm indoors. For details, see the following threads but the standard disclaimers apply. High pressure air is dangerous so proceed at your own risk.
Open the porting up...barrel port to 0.170". Be sure the deburr the hole inside the barrel afterwards. A 1/8" diamond ball in the Dremel works great.
And the same 0.170" for the nose of the bolt probe. Best way to do this with a hand drill is to use a numbered drill set and just step up one size at a time.
Enlarge the side opening. A diamond or carbide burr makes short work of it.
For the valve, you can ditch the fiber filter and install a longer spring (spring rate does not really matter for a regulated rifle), and hog out the sides of the valve body to improve flow. Reduce the poppet OD from 0.371" -> 0.340". Drill out the valve throat from 0.194" -> 0.219". Taper the valve stem from 0.117" -> 0.077" in the region where air flows around it and up through the exhaust. Also radius the inside edges of the exhaust (in the throat) to reduce turbulence.
Or you can ditch the entire front part of the valve and put a small return spring on the stem, held with an E-clip. Just keep a small ring from the aluminum body to capture the -113 O-ring.
For the transfer port, toss the pathetic factory seal and cut a piece of icemaker tubing (0.25" OD, 0.170" ID poly tubing) to 0.100" - 0.110" long, This length will compress slightly when you fasten the receiver to the air tube and form a good seal. Or you can use a Crosman #130-036 seal, which is nice because it can be reused through multiple disassemblies/reassemblies. The poly seal will last virtually forever if you leave it alone but must be replaced any time you disassemble.
Since the QB79 has almost no plenum volume, it helps to hog out the tank block:
When it comes time to dial in the hammer spring tension to find the velocity plateau, it's helpful to remove the rear cocking pin to convert it to cock-on-open:
Having converted to cock-on-open, a #10-32x1.5" screw through the end cap acts as a rear velocity adjuster (RVA) to fine tune the hammer strike:
Another view showing a #6-32 grub screw installed for locking in the setting.
If you have a lot of preload on the spring, the bolt may rest too far forward to load a pellet, in which case you can cut a rear downslot to latch the bolt into. A 1/4" carbide burr is the bees knees if you have to do it with hand tools.
If you do all that, you can expect about 30fpe with JSB Heavies:
Or you can chunk even heavier lead at 34fpe:
This is all hypothetical of course.