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Labtecpower wrote:GGDT predicts a 200 J increase in power, with the same projectile weight and a barrel of 230 cm.

If you're going to shoot the same projectile though, you're going to need to have a sabot, whose weight should also be included in the calculation.

First try using only projectile weight in the smaller barrel, see what velocity that gives you.

Then try the larger barrel this time putting projectile weight and sabot weight, and check the velocity. Imput this velocity and projectile weight (without sabot) into this calculator and see how the energy compares with your first value for the smaller barrel.

So,

Who wants to be the guy to make a sine bar rifling machine with cutter heads for various types of pipe and tubing?

I have found that 12 gauge piston wads, when loaded piston side forward, make the best blowgun darts for a 3/4" copper tube I have ever used.

Basically, any kind of shuttlecock-type shape will work well for drag stabilization, but doesn't that drag slow down the projectile more over a distance?

Hence my comment on the rifling.

mark.f wrote:Basically, any kind of shuttlecock-type shape will work well for drag stabilization, but doesn't that drag slow down the projectile more over a distance?

Hence my comment on the rifling.

Rifling = barrel drag = lower muzzle velocity.

I think the fact that APFSDS fired from a smoothbore barrel remains the premier kinetic armour penetrator on the modern battlefield means that drag/fin stabilisation isn't hurting performance enough.

Basically, any kind of shuttlecock-type shape will work well for drag stabilization, but doesn't that drag slow down the projectile more over a distance?

Hence my comment on the rifling

shuttlecocks are designed to be slow... their base drag is huge and their mass is low - you need exactly the opposite thing



What you really need is a simple to produce, cheap and relatively low drag projectile design...

Check what I posted on the first page of this thread - it can't get any simpler than that...

POLAND_SPUD wrote:Check what I posted on the first page of this thread - it can't get any simpler than that...

Don't you think it would benefit greatly from a lightweight boat tail though?

well it's not that the two designs are that much different... if the round works as expected then you might as well add a foam boat tail (or a PVC bell reducer) and see what happends

Would increasing the bore diameter increase the muzzle velocity?

The correct answer is "It Depends".

If the bore is larger than the valve can supply, then it will launch SLOWER.

If the projectile is larger and heavier to fit the larger bore, then it will be SLOWER. Power falls off when the bore is larger than the valve orifice in most situations. Sometimes a slightly larger barrel is better. A barrel twice the diameter of the valve is very limited in power. My best power was with the 2.5 inch barrel on the high CV 2 inch valve and the GB barrel on the Marshmallow cannon with the 1 inch valve. A lower CV valve won't do as well with a larger barrel.

If the projectile is the same as you would have used in a smaller bore, but used a sabot to increase the area for pressure AND the valve can supply the volume needed to keep up, then YES.. Much faster.

For reference on my experiments on the subject, smashing AA batteries was the test projectile. In a 1/2 inch barrel the result was OK but nothing to brag about. In a 2.5 inch bore with a sabot, and the larger valve to feed it, the battery because a dust cloud.

Oh. References.. You bet..
http://www.spudfiles.com/forums/flat-aa ... 21827.html

2.5 inch barrel


1/2 inch barrel

Hey guys its been a while but now that theres a break in FIRST robotics I decided to break out the old golfball gun and log back onto spudfiles.

This is a really interesting discussion and it prompted me to make a projectile based on some of the ideas, its 5/8" steel turned down to a teardrop type shape, its about 4" long and weighs 100 grams. I still need to design and make a sabot to fire this from my GB bore cannon, my valve seats directly to the barrel so i shouldnt be losing any power.


I think I can probably figure out the sabot but Im wondering whether you guys think welding small fins to the back would be a good idea. My plan is to take some highspeed footage and see if it tumbles, but I think some discussion would be interesting.

I would first try it this way. It would be great if it works this way without tumbling.

jor2daje, good to see you here again

Labtecpower wrote:It would be great if it works this way without tumbling.

I doubt it will, as it's made of the same material and the CG would not be forward enough. The Tallboy needed fins and it had a heavy steel warhead coupled with a lightweight alumnium tail.

It would be cool to see some high speed footage of the tumbling

You need to track the projectile over a significant distance to see it tumble so it won't look anything likethis...

sorry didn't keep up with this, but what the one guy said about using a sabot on the AA batteries is really surprising, i cant believe that simply increasing the diameter of the barrel and using like a pool floaty as a sabot to shoot a double A battery will go from a squashed battery to a pancake. now i look at my 1/2in barreled sniper cannon with a sigh...

The second one also uses a much bigger valve and air chamber, and don't use pool floaties alone as sabots, tech used it in conjunction with half a water bottle

The pool noodle was just to orient the battery in the sabot. The real sabot was the water bottle. The foam has a hole all the way through it. It did not push the battery at all. A larger barrel does require more air, so yes, a larger chamber and valve was used. This resulted in more energy used to push the battery.