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QB78 .177 MV- okay @ 600fps?

January 27 2017 at 2:17 PM

cobalt  (Login cobalt327)
YF12

New .177 QB78 bought 12-2016. Totally stock. Tested using a chronograph. First CO2 cartridge, getting ~600 fps w/Crosman Premier HP 7.9 gr, want to know if this is reasonable or abnormally low- I've seen reports of QB78 velocities being higher but little/no info on what, if anything, has been done to these guns to get the higher MV. All that I have done is clean the barrel prior to shooting it the first time.

Thanks in advance, Mark

 
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gubb33ps
(Login gubb33ps)
YF

Temperature?

January 27 2017, 5:55 PM 

How cool was the rifle (lets assume it was whatever the air temperature was)?

 
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Scot Laughlin
(Login classicalgas)
YF

Seems a little low to me, asuming your were shooting in air temperatures above 60F

January 27 2017, 9:03 PM 

There's lots you can do to bump the velocity up, the archives here are full of tune tricks, but this kit from Archers..http://www.archerairguns.com/QB78-XP-Tune-Kit-p/xptunekit.htm is a great place to start as a newbie 78 tuner.

 
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Sophie948
(Login Sophie948)
AR&P

According to the website Scott referred you to...

January 28 2017, 1:36 AM 

...a stock QB78 .177 should do about 625 fps.

But what the other fellow said: CO2 is highly sensitive to cold temperatures.

 
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cobalt
(Login cobalt327)
YF12

Temp

January 28 2017, 3:11 AM 

It was around 60-65 deg. F. when I tested the MV. If 625fps is about "normal" @ 75 deg. F, my gun is about where it should be, maybe a touch low, so I guess I can live with 600fps @ the temps I was shooting at. Thanks everyone.


    
This message has been edited by cobalt327 on Jan 28, 2017 3:15 AM


 
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gubb33ps
(Login gubb33ps)
YF

Easy enough to test...

January 28 2017, 9:41 AM 

...rifle won't cool down super fast, so if you set it in a warm room for a half, then quickly chronograph a couple of shots.

Good news is that all the QB's so far have gotten a little faster as the moving parts "mated up" and ran smoother.

 
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cobalt
(Login cobalt327)
YF12

No worries

January 28 2017, 9:28 PM 

According to a very reliable source, Steve Archer of Archer Airguns, "The muzzle velocity of QB78-type guns varies by about 2 fps per degree Fahrenheit". So my gun is right where it should be. Guess I'll have to find something else to worry about!

Thanks for the help, everyone. I appreciate it.

Mark

 
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Steve888
(Login Steve888)
YF

Re: No worries

February 7 2017, 3:23 PM 

Yes, 600 FPS for a stock QB78 in .177 caliber is right on the nail for muzzle velocity at 65 degrees F.

And yes, my experiments showed a difference of around 2 FPS per degree F in temperature change.

Stephen Archer
Archer Airguns Inc.
http://www.archerairguns.com

 
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