qb78 slowed down.....

qb78 slowed down.....

Joined: December 18th, 2009, 3:35 pm

May 25th, 2012, 11:27 pm #1

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:
-HDD
-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
-Lightened Hammer
-New crown
-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

Bruce
Last edited by bruce.itterman on May 26th, 2012, 12:38 am, edited 1 time in total.
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Joined: December 18th, 2009, 3:35 pm

May 26th, 2012, 2:25 pm #2

It was definitely the hammer spring.

I tok out the 2 shims and the speed was around 600 FPS and due to the lightened striker I was not getting that once the gun warmed up. So it actually worked better when cool once it hit 75-80F not good. Fire a few shots to coolt he gun and speeds went up.

So I stretch the spring a bit and also added 3 shims to the hammer spring till I can replace it.

Gun was about 72-74 after putting new cartridges in it for the last test (Was inside also @ 73F due to air conditioning)
-Loaded the cartridges

-Fired a few Crosman Priemiers 722-726 FPS (about 16.65 ft/lbs)... so same as before
-Fired a few 18.1 JSB's and got 672-680 (about 18.37 ft/lbs)....

I left the gun outside to warm up to more normal temp but expect higher speeds them... The test above was within a few degrees F of the original test so similar results.

About 82F outside. left the gun outside to get upto temp.

Crosman Priemiers 775 (19 ft/lbs)
18.1 JSB 720 (20.8 ft/lbs ) just did 4 more shots 720 718 716 (fast right after the other) let it sit about 2 minutes took another and it was 721

Wow that is smoking! Suprised even at these speeds how quiet it is.. The hammer is the loudest part thanks to the hdd. Now I know why the shot count is about 30 depending on the temperature.

Bruce



Last edited by bruce.itterman on May 30th, 2012, 5:33 pm, edited 1 time in total.
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Joined: December 5th, 2008, 8:43 pm

May 28th, 2012, 12:04 am #3

why not get a 79 and use a 13ci bottle regulated ,around 100 shots at a more consitsant velocity , dependant on output pressure,
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Joined: December 18th, 2009, 3:35 pm

May 28th, 2012, 12:43 am #4

hind-site is always 20/20 also no compressor gear etc... and no place close to get bottles filled.

I am going to bulk fill co2 I think... for now.. maybe a qb79 or a AR2079A will be the next project gun if I want to play again..

I also wanted to see with 2 12g powerlets what was the most power I could get with only stock but modified components...

Bruce
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Joined: December 5th, 2008, 8:43 pm

May 28th, 2012, 1:16 pm #5

I use a stirrup pump and find it quite easy to fill a 13ci bottle from 1000psi to 3000 psi for 100 regulated shots, it is also self contained ,
I found co2 hard to come by where I live and very inconsistant, especialy in the winter , ,
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Joined: December 18th, 2009, 3:35 pm

June 4th, 2012, 10:39 am #6

cost of a $300 pump not to mention another 75-100+ for the 850psi regulated tank, hose and bulk fitting fro the qb. Especially considering its a $130 gun.

$400 will get a lot of co2

Might be better off with a qb79 from MM, and add a HPA tank and pump... I expect would not be much different in price. Too bad don't think I coudl get a qb78 from MM due to being in Canada.

Bruce
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