The QB79 Saga

Well you all know I have a QB79 it's been a good airgun, it was good but not good enuff.
I had some problems with it, it was ripping the bolt o-ring I rip 7 in like 50 shoots.
so I send it to my friendly modded Pete, He made it a hard hitting airgun,

but after like 200 shoots it stop working I took it apart 7 time and on the 7th time got it to work,
but only for 20 shoots, so took it apart 2 more times and found the problem the inner valve was seperating.
Yes! the end part was slipping off the metal rod.

So now come in my friendly parts Seller Eric from Scopes & Ammo he sold me a new oem valve. [ and a few other goods but for a other post ]
I instaled the new valve with the new O-ring kit from S&A and put it all back to getter, clean,lube.
I tested it with CO2 and it work great.

now I got it connected to my 900lbs hpa air tank, it has not lost lbs of pressure in like 28hrs
so my work is done I will do a test on it tonight I will shoot it from 3k lbs to 1k lbs and count how many full power shoots I get.
after I will re pump it to 3k lbs and instal my scope and sight it in.

I'm very happy my QB79 is back I love this thing.

Great news Frank. Glad to hear that it was a simple valve stem.

Thanks Sean. was not ezzy.

Well I got some testing done I tested it for shoot count and speed.

tested from 3000psi to 1000psi I got 77 shoots.
FPS was 789fps at 3K to 729fps at 1K now I don't know much about this, like speed and all but I'm sure that my pellet would not drop all that much with 50fps differance, corect me if I'm wrong.

took me 100 pump or the Benjamin to get it back to 3000K and to be honnest it's not al lthat hard. At 1K I made 50 pumps that got me to 2K psi and I did 2 times 25 pumps and got it to 3K it was a bit hard
But has a Mechanic I work harder then that durring the day at work. so I'm not going to complain about pumping. [ well for now, but some day I will have a scooba tank ]

so later I instal my scope and sight it in.

P.S. Don't forget my 13ci tank is regulated at 850 to 900psi so the only thing making the pellets go faster in higher presure is the speed of the air moving out.

Regulators are not "exact" and the closer they get to 1000psi, the less "exact" they tend to become, at least in my, and a few other people's experience. Stay above 1500psi and things should be more consistent.

Actually, the pressure is regulated, so in theory it should be a constant 850 psi.... If it was, the pellet velocity would be a constant as well.... The fact you are getting a higher velocity when the tank is full is due to "regulator creep".... ie it's putting out a slightly higher pressure at 3000 psi than at 1000 psi.... It's likely about 900 psi at the beginning, dropping down to 850 at the end....

You're not getting a lot of energy from the amount of air you're using.... ie the efficiency is quite low, about 0.8 FPE/CI.... Combine that fact with the higher velocity at the beginning of the string and I would suggest your hammer strike is quite a bit more than you need for the pressure you are running.... The QB 79 I tested peaked at about 1200 psi, so at 850 psi you could use quite a bit less preload on the hammer spring.... As you reduce the preload, you will use less and less air, but initially the velocity won't change compared to what you have now at the end of the string.... However, the velocity at the beginning of the string will start to drop until it is nearly constant throughout.... Eventually, the velocity will start to decrease if you lighten the hammer strike enough.... Personally, I would cut a coil off the hammer spring and see if you get more shots and a flatter velocity string....

Bob

I read you wonderfull post on your QB79 mod I even have it in my favorites,
But I would like to get more shoot counts but in a few weeks I will be getting or
modding my tank for 1500psi so I will need the hard hit from the hammer at that time.
I'm not getting many shoots I know but the airgun is working perfect, it has not work this
good in months.

I'm falling back in love with it. But I have a new GF called Benjamin Discovery on it's way.