I got my sons qb78 out as he wanted to do some shooting now that its warmer. Took a couple shoots and hmmm sounds too quiet... I know since the HDD install its has been much quieter but not this silent so I took a look. Also shot count was not better so I knew something was up.
The Rubber breech seal was split... np I had another as I order a couple HDD's one for me and one for my brother-in-law and had not got to his. Replaced the seal and it helped but still not perfect only 630-640 fps with the .22 cp's
Tonight I decided to pull the gun apart again as I suspected the hammer spring. Much to my surprise the Rubber breech seal was split again.... I quickly got a piece of water line and cut a new breech seal and shimmer the hammer spring with 2 washers. (Originally they actually slowed down the gun a fair bit)
Well gun is close to were it was before but no quiet there only 700 FPS with 14.3 CP's in .22
I happy there, might replace the hammer spring.
My son's qb78 has the following mods:
-Probed the bolt
-Valve port ported
-Washer cut for more flow
-piecing pin cut back
-4 holes in the end of the valve body for more flow
-Filter removed from valve body
-Tapered valve for more flow
-Polly breach seal
-Ported and debur'd barrel port
-Full hammer polish and debur'd
-Extra set screw in the bottom of the barrel band
Was getting 724 FPS @ if I remember 72F when I first did it. I'm good with only 700 FPS 15 1/2 ft/lbs maybe get a couple extra shoots.
Might actually remove the hammer spring shims and see how it does. If I get 650-680 and a few more shots might be a good thing with a 7 year old... need to bulk this as he is going through a fair amount of c02 now...
No I am thinking what would it do with HPA ;P but not going PCP/HPA yet