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Alright Guys, I'm picking up my materiels for my GB gun this afternoon and plan on having it done by tomorow, and since you guy's seem to be all about some pics/video/damage shots(who isn't?), I've got a video camera lined up for sunday...
Specs on the gun: 60" sch80 with sdr21 inner barrel, 36" chamber, coaxiel, barrel sealing piston valve, modded garden sprayer for pilot, and I'll be running 100psi to it, and it will be modded for vacuum assist...
I'm also building a second identical GB barrel, barrel only(for testing purposes), modded for vacuum. testing for what, you ask... I'll get to that in a moment...
Things I'm gonna be testing and paremeters used: 1.Range of GB barrel only with vacuum(26"inHg). 2. Range of pneumatic with pressure only(100 psi). 3. range of pneumatic with 100psi chaber pressure and 26" inHg of vacuum in the barrel. 4. effect of projectile position(in relation to piston face/barrel breech) on range using only pressure.
test 1.ggdt calculates the vacuum operated, barrel only, will have a range of 525ft at a 33degree angle.
test 2.ggdt calculates the pneumatic with pressure only at 757ft at 33degrees
test 3. ????
test 4. ggdt shows a decrease in range of a coupla feet per 6" change in projectile position. we'll see if it holds true.
All testing will be done from a "carriage" that will hold the barrels at precisely the same angle and will not allow recoil.
test #4 I'm doing because it doesn't appear that anyone takes into accout that sometimes a little "dead space" between the breech and the projectile allows the air to accelerate and develop it's own kinetic energy creating a "harder" initial push on the projectile... Maybe it doesn't make much differance, maybe it does, after this weekend we'll be able to quantify it though.
The golfballs I'll be using will all be new identical balls, with 1 square of toilet tissue wrapped around to tighten clearances.
Am I leaving anything out?
Also, what Angle should I use? 45degree, 35degree, 20 degree??? I've got a little more than a half mile clear range that I can make measurements and recover the balls, beyond that there is 2miles of woods, Not sure I could get that far, but i'd rather make sure they all land in the clear for measuring sake. I have volunteers lined up to mark impact points( they'll be wearing full motocross gear for safety) and a gps for measuring...
Any other theories, or tests you'd like to have quantified???
Anybody know where I can get a chrono for muzzle velocities in short order???
Last edited by jeepkahn on Tue Feb 10, 2009 6:33 pm, edited 2 times in total.
Very ambitious, hope you get it right the first time
I had done a few chrony tests in this vein with a 6mm launcher, the results are posted somewhere on the forum, it basically resulted in a loss in performance.
[quote="jackssmirkingrevengeVery ambitious, hope you get it right the first time
Thanx... When I get the build pics posted I think you'll see that with proper planning, a coupla hours is all thats needed to build a gun properly... BUt I'm also a middle aged OCD enabled overthinker...
Using standard parts, perhaps, but sadly I'm of the epoxy mindset, a versatile material but it turns hours of building time into days of waiting
I shave a lot of time in the piston dept. by "inverting" the cup so to speak...(it only works with coaxiels)
I'm no good with graphics so I'll try to 'splain it... I take a "cup" so to speak,roughly the inside diameter of the outer pipe, and cut a gasket to fit inside the cup snugly, then I cut another gasket slightly larger(good fit, not too tight) to fit inside the outer pipe, I then drill a hole through the whole lot and use a small washer inside the "cup" to hold the gasket, and the use a large washer(almost as large in diameter as the gasket) on the outer gasket, and then as a bumper I use a 3/4" tall x 1"diameter bumpstop (available in most hardware stores as a doorknob stop). In order, I take a 6mm bolt insert it through 1.smaller washer,2. inner gasket, 3. through the bottom of the cup,4.through the outer gasket, 5. through the larger washer, 6.through the bumpstop. 7.through a washer that fits inside the bumpstop, 8. install the nylock nut and snug it down. I then slide the "piston/cup" into the barrel open end first.
this design allows a much more efficient use of pilot volume and allows the barrel to come almost to the bottom of the "chamber", the effective thickness of the piston is about 3/4" because by putting the open end forward it allows the sides of the "cup" to provide stability without added weight or dead volume... the bumpstop also serves as seal around the pilot to help hold the piston open until all chamber pressure is evacuated. I'll get pics as this one goes together tonite and try to post them tomorrow...
Slight change in plans on the testing, gonna be too cold, and I decided instead of making a seperate barrel only I just overbuilt the pneumatic... It's now 114" barrel x 56" chamber...Couldn't Help myself
It inverted the order of my pics, sorry...
Update, not a double post, too many pics for an edit...
I just built a golf ball cannon similar to yours, and fired it for the first time Sunday.
The gun is a coaxial with a diaphragm valve, using a 3/8" QEV for the pilot. Barrel is 70 inches 1.5" SDR 26 sleeved inside of 2" sch 40. Chamber is 41" 4" sch 40. Powered with CO2.
All shots were at 25 degrees elevation.
10 psi: avg 123 fps, 133 yards
13 psi: avg 170 fps, 187 yards
16 psi: avg 206 fps, 240 yards
18 psi: avg 231 fps, 252 yards
20 psi: avg 253 fps, not enough room to shoot it.
4 shots were taken at each pressure. The distances are where the balls stopped rolling. Temp was 37 F. and the ground was frozen. Distances were measured with a GPS, and velocities with an F1 Chrony.
When it warms up it will be interesting to see how it does at higher pressures. I trust the valve up to about 125 psi
If I needed to buy a chronograph I'd get this one:
$80 for the F1 Chrony. Midway always ships fast.
Nice to see your speedy build went well
Definitely, 20 psi for a GB cannon is a bit of an insult.
in six hours I built the gb cannon, and the 2 smaller coaxiels, I've test fired all of them(dry) and the only prob with any is that I had to order a 3/4" qev for the gb gun, the garden sprayer works great for the smaller guns, but it doesn't flow enough to pilot a 4"piston ( whoda thunk it? lol), the 2 smaller guns are already with their new owners( they were precommisioned), my qev will be here monday... the gb gun will be more than safe up to 200 psi....
And D hall told me how to calculate for the vacuum in the barrel, and the nummbers look very promising for making sonicbooms...
What's your pilot volume like?
Remember that a piston only needs to move back 1/4 of the barrel diameter for maximum flow, anything more is usually waste (assuming the piston doesn't bounce) - so if you're trying to maximise efficiency - advisable if you've got the sound barrier in your proverbial sights - then minimising pilot volume is a great way to reduce valve opening time for a given pilot valve.
I was thinking that a coax had to have half the diameter for max flow, or am I remembering incorrectly... and are you asking the total volume behind the piston or the displaced volume by the piston during travel(it's not entirely clear what people are calling pilot volume, I always thought pilot volume was the volume displaced as the piston traveled)???
piston moves approximatelly .95 inches from closed to open( i know, a little overkill)... and when the piston is in the full open position, the remaining volume behind the piston is very small, maybe 1/3 of the displaced volume...
How'd you like my blowdart???
Last edited by jeepkahn on Wed Jan 21, 2009 6:32 pm, edited 1 time in total.
Whip a half inch ball valve on the two smaller co-axials and notice the difference in power over using the garden sprayer
I can notice a big difference in small bore cannons, using a QEV for the main valve, with a blow gun and a 1/4" ball valve. Sometimes ergonomics gets the better of me, but most of the time I go with power.
On both my 2" piston valved cannons I have a 1" sprinkler valve for the pilot.
out of curiosity, how much do your pistons weigh??? mine are less than 1oz.. about 24g to be exact...
I'm sure qevs work better, but $5.69 for sprayer and $25 for a qev, I can live with the slight diff in performance on the small guns, on the big one I'll spend the $'s...
the smaller coaxs will seal and fire with 3psi, so I'm fairly happy...
My pistons weigh probably about 50g or more...it doesn't really matter too much. Even if there is a delay in actuation, it could be benficial but I doubt it'd make much difference at all. Probably the same amount of difference as having a linear cannon to an over-under cannon.
Though there is a line somewhere, where piston weight will effect performance noticably.
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