SpudFiles

Using this gun, but with extra 1/4" QEV and a new blowgun, and with some pvc bolted on to prevent rotation:


So I set the chrony up when the rain stopped...


Results, 30 bar, 16mm marble:





270 m/s!

Thats more then I expected. GGDT predicts 210 to 240, 250 with optimal assumptions. Isnt it that GGDT is made for lower velocities and that it doesnt calculate right when coming near the sound barrier? Or is my chrono incorrect? Could also be muzzle blast.

Anyways, 972 km/h is just awesome (and over 900).

I (and others) thought the same thing about the muzzle blast. Then I "dry" fired my bb experiment over the chrony.

There were no errors, and no readings.

If you don't mind, try a dry fire and see if your chrony has the same results.

It could be that the distance requirement is more to prevent reading unburnt powder etc. that is also leaving the muzzle of a firearm.

The dry firing is a good idea! I'll do that soon.
Yet most people having errors from muzzle blast get WAY to high readings. This is just about +10~15% from calculated speed.

I don't have a sight or scope on the gun (yet). Until that time I can't try doing it from a distance. I just can't really aim, only "estimate". And chrony's are not designed to take hits like that...
I could try to shield the chrony though.

Gippeto, I've dry fired most of my guns over the chrony at near point blank range, the only ones that show any err's or misreads are the larger than 1.5" cannons, and I'm pretty sure it's the condensation plume causing it then, the smaller bore stuff never misreads a dry fire...An FYI, when using it for PB guns, you're supposed to be about pointblank, according to the company that makes them.... Another tip, instead of using the steel uprights, use long bamboo skewers, that way if you hit the uprights you don't destroy the chrony...

Jeep, I'm getting more and more certain that my chrony is not registering anything other than the projectile. I'm still interested in a "cross section" test where others are reporting their independent results.

If everyone does get the same "non" result, then perhaps some theory's about what it takes to hit sos with room temp air need to be adjusted.

With 400psi air, I've gotten chrony readings over 1200fps with a .12gram airsoft bb.

http://www.spudfiles.com/forums/x-1-1-m ... 16607.html

The sticks are a very good idea.

will i have said that before
ggdt is good software but ,again like what i said the data yo get from it (velocity) are not very accurate

Sweet

though technically, a chronometer is a watch, I think you mean chronograph

Gippeto, You may have hit SOS, depending on what sos was that day(everybody talks about temp effecting sos, but they forget that humidity, and barometric pressure play a part as well)...

Everybody seems to think that the only way heat is generated in air is from compressing it, with enough pressure velocities in the muzzle can well exceed the sos due to "choke heating" and the LOCAL sos(inside the chamber/muzzle) can be higher than atmospheric sos, so that even though the velocity is over the atmospheric sos, it's not moving faster than the local sos(this is why evacuating the barrel is more effective in subsonic guns than super/hyper sonic guns, the vacuum raises the local sos or mach# if you will)...

Muzzle blast in airguns is a non issue unless it is humid and you start getting condensation plumes, if you look at the paintball kit for chronies, the muzzle is pretty much point blank...

And wouldn't it be pretty easy to just fire 5 shots,start at 1ft and moving a foot each time, should settle it pretty quick...As long as you have consistent ammo and pressures...

jeepkahn wrote:Gippeto, You may have hit SOS, depending on what sos was that day(everybody talks about temp effecting sos, but they forget that humidity, and barometric pressure play a part as well)...

Everybody seems to think that the only way heat is generated in air is from compressing it, with enough pressure velocities in the muzzle can well exceed the sos due to "choke heating" and the LOCAL sos(inside the chamber/muzzle) can be higher than atmospheric sos, so that even though the velocity is over the atmospheric sos, it's not moving faster than the local sos(this is why evacuating the barrel is more effective in subsonic guns than super/hyper sonic guns, the vacuum raises the local sos or mach# if you will)...

Muzzle blast in airguns is a non issue unless it is humid and you start getting condensation plumes, if you look at the paintball kit for chronies, the muzzle is pretty much point blank...

And wouldn't it be pretty easy to just fire 5 shots,start at 1ft and moving a foot each time, should settle it pretty quick...As long as you have consistent ammo and pressures...

I've been playing with GGDT and small barrels on large valves and have been getting interesting results. I will need to chrono a few shots to see if GGDT is even in the ball park. When I get into the realm of warning the barrel may be choking the flow, some of the speeds are well above the SOS. For example on my 1 inch valve, using a 3/4 inch screw on barrel 10 feet long, a 2 gram projectile (miniature marshmallow) and 200 PSI puts the speed well above SOS. I need to buy a chrono.

I've seen the same thing if I screw on a golf ball barrel on the not yet built 3 inch 200 PSI cannon. I should have results this fall but I like what GGDT predicts.

Using a light projectile about 1/2 the mass of a golf ball and using the following;
Gas Air
Temp 80 F
Pressure 200 PSIG
Outer Diam 12 inch
Inner Diam 3.3 (QDV coaxial style)
Length 22 inch
Valve type Barrel seal (QDV not listed)
1 valve
Efficiency 45% (probably low but using default)
Piston Diam 3.1 (had to make it bigger than the seat for GGDT)
Mass 24 oz (wild guess at just under a couple lbs)
Vent diam 1 inch (needed something for GGDT here)
Pilot volume 20 in3 ( the bumper trigger area )
Bore 1.59
Length 164 inch (long barrel also tried 120 inch (10 ft))
Friction 0.1 psi (roll in projectile)
Mass 20 gm (foam ball. GB is about 46 gm)
Diameter 1.56
Initial position 0.

This gives a muzzle prediction of 1511 FT/S.
I hope to try it late this summer. The heavier GB will still be subsonic 1085 ft/s isn't bad.

jackssmirkingrevenge wrote: though technically, a chronometer is a watch, I think you mean chronograph

Actually chronometer and chronograph are both incorrect, though both are in "common useage" for this application. "Chrono" is time of course, "meter" means measurement and "graph" means measures and record (as in plotting a graph or storing the value digitally).

A "shooting chrony" does "meter" time but it doesn't display a time measurement, it displays a value related to the reciprocal of the measured time. Not sure what that would be called, a "one-over-chrono graph"?

Meter, graph.... who cares? Lets just stick with "Chrony" then.


I've got news! I dry fired two shots.
Nothing was recorded.
Yet a white muzzle blast was clearly visible. This is partly oil, my fridge compressor spits out an oil vapor along with the air. This is kinda nice though, as pistons and valves automatically get lubed up and this oil doesn't eat rubber and plastics like orings and sealing faces.
It seems like I really shoot marbles at 270 m/s...

I conclude that GGDT is counting the SOS limitation too heavily and thus is inaccurate with high powered launchers, firing well above the 200 m/s.


Last step now is to shoot from a distance, with a well-shielded chrony. I'll probably just put a rock in front of it as I have a detachable wired display.

I can't fire like 10 shots in a row due to me living in a residential area and disturbing the whole neighborhood from my backyard.
To prevent cops showing up, I usually fire a few shots and then get back inside.

I hope I can come over next saturday and get a reading on "Old Shatterhand"


If this thread is on the right track I might have found the reason for the big boom I got when trying to diesel it...

Oh dieseling! I almost forgot. We should check that out once you come over.
With a bit of luck we could possibly confirm a supersonic shot... sweet.



Dutch conversation, too lazy to send PM:
Neem ook gelijk je fridge-stack en die engelse sleutel mee. En als ik voor projecten in de zomervakantie een BSP tap kan lenen zou dat fijn zijn.

Choking... Key word here, I'm not sure ,you'd have to ask D-hall about his usage of the word in ggdt, but choke normally means in gas flows that the combination of pressure and limited orifice size cause mach, or faster than mach, speeds to be achieved, so technically, if it ain't choking it ain't going faster than SOS....

http://en.wikipedia.org/wiki/Choked_flow

psycix wrote: To prevent cops showing up, I usually fire a few shots and then get back inside.

are you from the irish resistance

can you try damage 2 3/4 " plywood ?