I bought a new QB 79 just to play with and when I got it I put a few pellets over the crony, 7.9 CPL's shot at 596 averages, not bad I thought since it's rated at 600 fps.
I wanted to shorten the barrel like my QB78 so I removed the barrel and chopped 5 inches off it and here's what I found.
The bore is off center almost .080 thousands 5" from the muzzle and this is the second barrel I found this problem in a QB rifle. My little short barreled 78 was the same way and it has an old Gamo Shadow barrel fitted to it now, because the stock barrel was bad like the 79 barrel...
Well I made up a .20 barrel for it and while I was at it I tore down the rifle and went to hopping it up. The bolt is probed, the valve spring is stock no spacing, highly modified the valve, and highly modified the barrel also.
The barrel was drilled for four gas holes with a groove around the diameter to feed them, the valve was cut away because it's a bottle fed rifle and don't need a piercing pin so I cut that off and opened up the valve a large amount. I matched the feed hole between the tube and the bolt and enlarged it slightly, next thing I am going to do is enlarge the hole even more and spot face the tube and the bolt housing and make a bigger diameter seal for it so I can make the hole in the valve the same size as the hole in the tube. This should feed the barrel with a greater volume of gas because of the restriction of the valve hole.
I have some new .20 barrels coming so I can make a longer barrel to take advantage of the increased gas charge, it needs a longer barrel but all I had was a 17 inch barrel to work with and with the cut out for the loading of the pellet you end up with about 15 inches of rifling.
Well here's some pictures of the valve and barrel, and by the way it now shoots 11.2 FTS's at an average of 720 and a longer barrel with bump that up a bunch, I am getting a huge amount of fog out the muzzle so I know the gas is not all used to the maximum and it is LOUD !!!
