Picky co2 adjustments
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Topic: Picky co2 adjustments (Read 192 times))
Ribbonstone
GTA Senior Contributor
Posts: 12030
Picky co2 adjustments
«
on:
June 28, 2022, 02:51:08 PM »
You’d think tuning a CO2 rifle for low or high power would be super easy….just reduce or increase the striker spring and you’d be good.
IF you’re picky….that has some side effects. What caliber doesn’t really matter, the cooling issue is the same.
“Hot Rod” co2 rifles, adjust with spring tension up, tend to declining velocity as they cool off. Even if the outside air is stable, the rifle will self-cool from co2 expansion/ejection as you shoot. Doesn’t matter what caliber, it’s more about the co2 used for each shot and how rapid your pace of shooting.
Noticed that as the .25 QB rifle cooled off (self-cooled from co2 being used) from 87F, that it shot faster and faster down to where I stopped at 75F….which makes for vertical “strings” on paper.
Wouldn’t matter if I wasn’t trying for tiny groups. Wouldn’t matter if I just waited long enough between shots to get it back to one stable temp.
Striker hits valve stem, valve opens, but how far/how well it opens depends on the pressure holding the valve closed. With co2, pressure/temperature are functionally the same; warmer is higher gas pressure than cooler.
Just using factory spring/factory spring tension , this one wants to work it’s best at about 75F...which isn’t going to happen in the summer unless I hide in the house.
Being picky, I want to be able to shoot with more of a temperature/pressure range with the least velocity change.
Get this part straight...I don’t want the rifle to shoot fast/powerful. Want it as a .25 back yard rifle, so want to keep it slow...but don’t care for the velocity change just from self cooling.
It is possible to do, have managed it on other co2 QB’s. No noticeable velocity change from something like 90F down to about 80F. More about self-cooling, rifles adjusted like that allow a more rapid pace of shooting before the self cool to where velocity starts falling off. (“Rapid” in the single shot sense).
Will go back into the rifle, increase the spring tension (to get it working better at higher temperatures/pressure) but also strangle the transfer port to keep speed/energy around 15-14 foot pounds.
-------------
Did get out to shoot it at 25 yards. Let it get temp.stable (just leaned it against the fence for 15-20 min). Over cast/rainy day, so only 87F. Paying attention to a thermometer strip stuck on the gas tube, all shots were within 2-3 degrees of each other.
DSCN0302
by
Robert Dean
, on Flickr
I still refuse to measure “rip paper”.groups..but a visual check of the groups were good enough for this.
Definitely need a bit more right wind age. Not knowing which pellet would shoot well, I just ignored that while pellet testing.
Crosman pellets are not this barrel’s favorite and were slower than redicted. Were not the favorite when this .25 was a PCP either.
Crosman pellets would shoot 38 fps faster at 74F than they do at 87F.
Would have thought the 34gr. JSB’s would be under-spun at this low speed...but they shot about as well as the lighter JSB’s.
No significant change at short range from when it was a PCP at 2X the speed/4X the energy.
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Louisiana
Goose
Sharp Shooter
Posts: 861
yes
Real Name: Jeff
Re: Picky co2 adjustments
«
Reply #1 on:
June 28, 2022, 05:37:51 PM »
Thanks for the info, Robert.
I've noticed that velocities do tend to rise a bit as the charge of co2 is consumed and that explains it perfectly. Next time I pass by a pet store I'm going to pick up a temp-strip for the QB78.
Stay safe,
J~
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Western NC
"There are two types of people. Those that are interested in controlling others and those that aren't." - R. A. Heinlein
Ribbonstone
GTA Senior Contributor
Posts: 12030
Re: Picky co2 adjustments
«
Reply #2 on:
June 29, 2022, 12:09:31 AM »
Considering how hot it’s getting….there are terrarium peel and stick strips that go up to 105F…..most aquarium ones top out at either 93F or 97F.
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Louisiana
Ribbonstone
GTA Senior Contributor
Posts: 12030
Re: Picky co2 adjustments
«
Reply #3 on:
June 29, 2022, 11:55:29 AM »
Should have added....not that you need 105F, but it's easier to read the temp when it is in the middle of the strip rather than at the far ends.
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Louisiana
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Picky co2 adjustments