SpudFiles

No, I don't have any pics of the chrony read, and conditions may or may not have affected the local SOS, but there was a significantly differant report on this shot than normal...

The gun: The decimator
Charge: unregged co2 on a 88degree day...
projectile: 1" superball
chrony reading: 1265fps
air temp: 88f
humidity:36%
elevation:about 1200 above sealevel

the superball is slightly larger than 1" and the type k barrel is .997, and i had to liberally lube the barrel and ball with silicone spray .... The iron chambers were quite hot to the touch even after filling(been sitting in the hot sun all afternoon), and I waited about 30-45 seconds to fire after filling...

The chony was screwed down to a large steel plate that was ratchet strapped to the picnic table, and the shot was 14" above the "eyes" and 6.5ft from the chrony...

I'm pretty sure it was above SOS, but I'll let the experts give their opinions with all the given parameters of temp and other atmospheric conditions... And no, I don't know the exact weight of the suprball but I'm sure someone here has weighed one and can give a close weight...

You'll probably have rag chime in, and seen as he likes to burst bubbles he'll probably tell you that you were 20 fps below the SOS.

Great job though!

GGDT tells me that with hydrogen my next cannon will go at least 200fps over the SOS, and be knocking on the door with air.

Nice work!

yeah and thats with co2 which has less performance than air or other gasses maybe one day you can get some hydrogen and get some amazing results i agree with nonickname someone will pop their head in here to discredit this

I'm not saying it's wrong or impossible, but CO2 will give you a hard time before it goes supersonic.

However, it's purely circumstancial.

Although the SOS on that day will have been about 1150 fps, you won't have beaten it - because I find it incredibly doubtful that you actually hit that velocity.

The reason is your propellant gas. Regardless of the pressure, CO2's speed of sound is a little under 900 fps (dependent on temperature). You'd be achieving Mach ~1.4 (or more) in the propellant gas for that to be possible, and given the dynamics of CO2, that's just not going to happen.

I'm afraid this is almost certainly a dodgy chrony reading.

While I can buy that an air powered cannon with just the right design could hit those velocities under exceptional circumstances, it's just not going to happen with a CO2 powered one.
CO2 is not a gas that is suitable for supersonic attempts.

Also, as I have said before, a different report is no guarantee of anything as regards velocity.

EDIT: Also, I don't do this because I like to burst bubbles. I like to be accurate, rigorous and scientific.
Bursting bubbles is just a pleasant side effect.

I'm not going to say that I might not have missed the mark, but I find it a lil odd that I'm consistently getting readings of over 1000fps with wooden balls, and close to 900 with steel balls, using co2, and I did some comparisons between hpa and co2 and even though the physics say otherwise, I'm reading faster with co2 than with hpa by about 8-10%... And when firing real guns over it 9mm and .40 cal It's reading with 3% of advertised velocity of the bullets being used...

To be quite honest, I wasn't expecting to go SOS with co2 because of what I know or thought I knew about co2 and it's dynamics, so I do question if there are factors coming into play that could indeed produce a faster than SOS shot with co2... ie, chambers being opposed, projectile initial position( about 5/8" from breech), the fact that the iron chambers were prolly about 95f when I fired, the momentary vacuum created from the piston retracting from the cross...

Would I bet money that I went SOS, prolly not without a differant(less fallible) way to measure fps...

All things considered I do THINK it went SOS, but logic say's it shouldn't have....

maybe some of the others like D-hall might chime in and offer explanations of why it may have or definitely did not...

I DO agree with Rag that it shouldn't have, but the chrony is too accurate the rest of the time for me to easily dismiss it...

I think what you missed is the hot barrel. The air the projectile plowed out of the barrel being hot and then heated more by compression heating may have given you the edge for the projectile in a light gas filled barrel. I think your results are most likely correct.

My 3 inch is primarily being built for an attempt at supersonic on air. GGDT predicts I should hit it with a good margin to lay aside doubt. I'll use a wire break to clock it. The 3 inch valve outlet will venturi down to 2 inch for a high COF. GGDT is predicting speeds at about 1.5 at 200 PSI.

I think you hit it dude.. Congrats.
:wav:

jeepkahn wrote:even though the physics say otherwise, I'm reading faster with co2 than with hpa by about 8-10%...

What are the relevant pressures of each? I assume you have a pressure gauge you can refer to.

I still suspect chrony error. Bear in mind that it's pretty much a given that higher pressure CO2 cannons will have solid CO2 in the muzzle blast. This could easily mess things up.
Also, CO2 absorbs strongly in the infrared spectrum (much more strongly than water vapour IIRC), so it's probably a much bigger problem for chronies than air.

The momentary vacuum created from the piston retracting from the cross

Doesn't exist. The piston is pushed back remember, not pulled.

~~~~~

@Technician1002: GGDT has known issues around the supersonic region. I've had it predict over 500 m/s for some projectiles (which is actually in excess of the particle speed in air, so impossible). The end result barely scraped 325 m/s.

Results in GGDT that exceed the SOS in the propellant gas should taken with extreme scepticism.

And I didn't miss the temperatures - they simply aren't hot enough to make that difference.
Also, increase in propellant gas temperature and SOS almost certainly means an increase in ambient temperature and SOS - the two cancel out!

I rechon 6.5 feet is still too close to the chrony, any chance of moving it back a little further?

the piston on this gun is a toolie, the part that is acted on by releasing the pilot is 5" of 3/8"threaded rod away from the piston head(what seals the barrel), the piston "head" actually is 3.53ci of dead volume that is "pulled" out of the breech area... so in this case the piston dead volume is not pushed out of the way... and the hpa was regged to 850psi....the co2 was unregged so you could prolly tell what the pressure was by temp. better than I could tell you, because my gauge is on my regulater and wasn't used for the co2 shots....

Also I just read some info on co2 and temps and if I was getting liquid into the chamber and the chamber was above 87.9f then I was pushing in excess of 1100psi... but I'm sure someone could shed more light...

and relative to mach #'s and pressures, isn't it possible that high enough pressures have the ability to force a gas to well exceed it's given mach #... And with a chamber barrel ratio as high as I'm running 1.5:1, c:b, and a starting pressure of 1100psi the projectile is leaving the muzzle with 850psi still in the chamber...

but I'd love for this to be explained better than I can...

Firearm bullet from a known supersonic round in a sabot and compare it to the books'o'truth?

jackssmirkingrevenge wrote:Firearm bullet from a known supersonic round in a sabot and compare it to the books'o'truth?

I'd consider something along those lines, but even a 9mm ball round prolly weighs more than a superball... a pendulum would work but requires mucho math to calculate results...lol...

I will also add that the report was not singular, it kapowed when i pulled the trigger, and keracked as it was leaving the yard... if that helps...

A typical 9mm slug weighs less than 8 grams, A foam sabot would weigh next to nothing, I'd consider it

jackssmirkingrevenge wrote:edit: double post

Shame shame....

Actually I am going to fiddle with bullets/sabots this weekend to try and settle this... Too bad chronies are so fallible...