Help me understand how CO2 works...

[stretch]

I have a general understanding of how regulated and unregulated HPA works, but not CO2.  Do typical CO2 rifles (like the Fusion) have a similar bell shaped power / fps curve as unregulated HPA?  I've seem 10m competition rifles that use CO2 so I'm guessing those rifles use regulators or CO2 has some property I don't know about that self regulates the gas.

Also, what's the coldest air temp that you will still get "reasonable" performance?  I understand that -hotter- weather is better, but when that's not possible how cold / cool will still (sort of) work?  And if shot count / velocity are effected, is that effect consistent?

[Toddspeed]

Look at the chart below, it shows that at 80 degrees C02 gives a steady pressure until the drops off due to being exhausted. No regulator is needed at that temp. According to the yellow box you see that pressure drops significantly with temp. I have used my QB78 outside at 50 degrees but it’s so different than at 70 or above that I really choose not to use it, it’s just no fun for me. Also notice that at 90 degrees and above it starts with a spike, then levels off for a while before dropping until exhausted. This makes it difficult to get consistent groups at 90 and above.
The regulating nature is due to the gas being in a liquid state and vaporizing gradually.

[Marc In Iowa]

I experimented using a CO² rifle for highly accurate Field Target use.

If I took 4 deliberate shots in a row (like you would, aiming carefully), my Extreme Spread was a mere .5% variation. I've never seen ANY regulator that was that good. The best are around 2%.

But.

The 60 shot Field Target course I shot lasted something like 4 hours to get thru the shooting lanes.

Shot count was NO problem. My 177 Hammerli 850 gets easily over 150 good shots from a single 88 gram cartridge.

However, over the course of 4 hours, the temperature went up over 20°F from start to finish. By the end of the match, the POI for my pellets was "anybody's guess." My range card was useless.

You aren't allowed to re-sight your gun during a match. If you were, and could adjust your scope every 10 shots or so, CO² might make a darn fine, entry level FT rifle.

For casual plinking and paper punching (or even small game, I suppose), if you can re-sight to adjust for changing temperatures, my CO² Hammerli is a very fine, accurate rifle.

I've used it as low as 50°F with no problem.

I see in your signature you're looking for a quiet backyard shooter. That Hammerli 850 fits the bill (within described limitations). We refer to it as "luxury shooting." Very nice trigger, great inexpensive magazines ($8, flawless feeding), added a $40 barrel accessory plus some tape (super quiet) and <1" groups at 55 yds, big shot count, no pump needed and you can learn to refill the 88 gram cartridges for <$1/ea.

I don't shoot it as much anymore 'cause I got a great PCP (177 Bantam) that gives me 80 accurate shots from only 60 strokes of the hand pump and isn't temperature sensitive but for a lot more money.

[Ribbonstone]

OK...it's been posted...will refer to that graph.

1. Plus points for the vertical red line to mark off 100% fill.  Lets just keep to rated fills, and ignore all that stuff to the right as things we are nver going to do (the quick pressure rise should be reason enough).
2. Are some nice wide "flat lines" (horizontal lines) in the ranges below 90F where pressure doesn't really care so much about fill % (how much co2 is left in the system).  That's low pressure/lower energy areas, but pressure stays pretty stable so long as the temp. is stable
3. From 90F on up, there really isn't a large area of uniform pressure.
4. Graph is for sealed containers of co2...and an airgun is a sealed container (with a little burp of gas let out now and again to shoot a pellet).  It's just an oddly shaped sealed container.


Co2 is a gas that is liquid if put under enough pressure.  Are some other gases that show that trait as well, but they aren't used as airgun propelants.

The ratio of gas to liquid changes becasue of temperture and volume (the percent of rated fill can be considered  a volume measure), even while they are sealed containers. Same weight of gas, just that some is gas and some is liquid. Warmer temps, and more gas and less liquid (which raises the gas pressure).  Cold weather,the less gas.

We upset the closed system by tapping off little burps of co2 with each shot.  So basically we are decreasing the amount of co2 gas in the system with each shot(changing the charts fill %).....but co2 is always going to try to make for a stable ratio : burp out a bit of co2,the liquid cos converts to gas.


Self Cooling:

Didn't mention that the conversion from liquid to gas gives up heat (it's a cooling reaction, like water evaporting on your skin) and so does decompression.  So each shot cools the system a little bit.   Given enough time,and it will get back to ambiant.

This is tougher on rifles that use more co2 per shot than rifles than use less ("hot rods"vs match speed rifles) simply becasue there is more decompression and liquid-to-gas conversion going on with each shot...so a larger temp. drop.

[Wayne52]

Co2 is very consistent for plinking indoors you need to know your gun to keep the shots consistent though.  The new replica guns can use it very fast if you don't give them a long wait to warm back up but it's not a big issue on all steel guns that release the pressure into a steel tube when they're pierced.

[Toddspeed]

If you’re looking for the easiest way to get a quiet backyard gun, I suggest you keep the Discovery and make a home made silencer if that’s legal where you are. I made one from plastic sprinkler pipe and it quiets my QB78S very well. If you are trying to get away from pumping that Discovery, isn’t it a duel gas gun? You could just switch to bulk CO2 and again you get to keep the gun you already have.

[stretch]

Awesome info, especially the chart!  Thanks very much!

I'm gonna give an Umarex Fusion a try later this week. 

Yes, I've got an eye on the Hammerli 850.  A bit more than I'm ready to spend, but that could change...

I also looked into modding my Disco for quiet, backyard shooting.  Not sure that it will cost less than buying a Fusion, but that's also a possibility.

[starlingassassin]

this would make your disco quiet,

https://www.ebay.com/itm/crosman-MONOCORE-ldc-1377-2240-discovery-2400kt/183622162219?hash=item2ac0bbd72b:g:VZcAAOSwtfhYre3p:rk:1:pf:0


i have bought numerous ones from this seller.

and at $37 shipped its easy on the wallet. (make him an offer of 32 and he will usually accept it  )

[Marc In Iowa]

... Discovery ... switch to bulk CO2

I bought the fill kit necessary to fill my Discovery with CO². It's not so easy to get a "good, full, fill." Why not?

To get a full fill, the "air" reservoir needs to be colder than the bulk CO² bottle. I ended up wrapping a soft cold pack from the freezer around the Discovery air tube before filling.

It works, but takes a little experience to do it right. In the end, I never found it satisfying.

I do love the Hammerli though. 🙂

[Ribbonstone]

Still shoot a lot of co2, although I also gave up on bulk filling. Still like 12gr. for low speed rifles and like bottle rifles (just about the only one left is the QB79), but not so hot on the cost of 88gr.

If the tempeture is pretty constant...and on many days, the temp doesn't vary much in a one or two hour shooting session....it really is self regulating....and works great for me in the lower power/short range applications.

But I pretty much leave co2 for outside/field use about the time of year I really need long sleeves and a sweater (by the time I need a coat, they're pretty much in storage) or doing indoor duty.

Marc:

IF not moving around, like shooting from a bench or from a lawn chair, just leave the Disco teathered to a tank.  It won't be a great power producer, but it can be pretty consistent.

Not a Disco...but likely becasue this rifle was made to operate at 1500psi, the results are likely closer together.






Last summer,I whent ahead and bought the best 12gr. rifle I could find....which was bascly a Hammerli 850 at heart, just kind of adapted and dressed up (AirGun Depot had them as one of their offerings).

Basically, I fell for a pretty face....again....but it actually does shoot freakishly well.

DSCN1228 by Robert Dean, on Flickr