Unexpected chrono and accuracy results with the QB78

[Goose]

Co2 cartridges were in short supply around here until recently, so using one instead of two at a time had become an option.  Most of our shooting sessions aren't that long anyway.  The rifle has been toned down to an average velocity of about 500-fps.

So an experiment to find what results it would offer was in order.  The temps have been around 70-degrees.
 
One full cartridge inserted with an empty didn't give many shots and not much in the way of velocity.  All that empty space really does reduce the performance.  Twenty-three shots at an average of 447-fps and a total yield of 149 ft/lbs was a disappointing find.  Half-inch groups at 10-yds were the result.

Using one cartridge with another full, but unpierced cartridge yielded some very surprising results.  (Placing a small nut over the piercing pin in the end cap kept it from being pierced.)  That gave over forty shots at an average of 508-fps.  A total of 325 ft/lbs.  Much better than expected!  The velocity did not start out slightly low, rise some and then fall off gradually as is common with two cartridges either.  The initial velocities were highest and then gradually fell off.

Shooting with two full cartridges had been generating seventy-two shots at an average of 502-fps.  That's a yield of 572 ft/bs.  Seems that it's more efficient to use one cartridge at a time.  A 650 ft/lb yield and over eighty shots at more consistent velocities from two used separately is probably the way to go.

A slight increase in accuracy was also noted.  Thinking that the more consistent velocity is a contributing factor.  1/4" groups at ten yds with flea-market find Crosman hunting pellets are now normal.  This has me thinking of making up a flat-ended cylindrical dummy cartridge on the lathe and using it as a substitute going forward.

Thoughts?

Cheers,

J~

[aluminumfetish]

They are fun rifles for sure. The 78's and the HPA79's. I tried using one cart in my 78 a few times but it did seem a bit anemic. I have my 78 cut very short and I mostly shoot in the basement or in the garage. I wonder if a solid plug (oak dowel ?) to take up all that space in there might yield better results. I may have to try it next time I shoot her down to empty.