# Tube Volume and Dart Velocity



## NightKnight (Dec 5, 2012)

I was thinking about dart velocity the today.... It struck me that that the additional air volume required to fully accelerate a dart in a .625 vs say a .40 may be larger than one would intuitively think. So, I did some calculations. All of these volume calculations are assuming a 5' blowgun.

.40" diameter - 7.542 cubic inches

.50" diameter - 11.785 cubic inches

.625" diameter - 18.415 cubic inches

Volume difference (increase):

56.25% - .40" to .50"

56.25% - .50" to .625"

144.16% .40" to .625"

Thus, not counting barrel friction, a given shooter will have 60%+ higher velocity with a .40" barrel over a .50" barrel, and the same with the .625. Now, friction in a barrel increases as the square of velocity so the gains will be reduced somewhat as the speeds increase.


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## NightKnight (Dec 5, 2012)

Now, the aerodynamic drag on a long cylindrical object is directly proportional to the cross-sectional area of the object. The math above applies here too. This means that compared to a .40" dart, there is 56.25% more drag on a .50" dart, and 144% more drag on a .625" dart. So, it seems to me that for longer distance shooting a .40 would be a much better choice, all else being equal. But, I don't know that all else is equal. I think that dart weight would also play a part in how long it maintains its velocity over distance.

It seems to me that the best option would be a .40" dart made of high weight material, like Tungsten.


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## cjb4u (Dec 10, 2012)

That is my understanding. But I think drag is a little different while in a tube considering that no air is passing around the projectile / cone so after the first resistance of getting the dart moving the air amount is pushing out of the tube in front of the dart. So just a quick question would the air not exit the tube of a larger calibration easier then a smaller caliber? Or would it not just equal out? Kinda like water pushing thru an empty hose with the air that comes first.


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## NightKnight (Dec 5, 2012)

Sorry, I meant the aerodynamic drag after leaving the tube. I am kind-of mixing concepts. The drag would only affect the projectile after it left the barrel.

As for your question, A larger volume of air will exit through a larger diameter tube more easily, but at a lower pressure. I believe that pressure is your friend when it comes to blowguns.


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## neondog (Dec 29, 2012)

Dart acceleration results from pressure differential applied to the surface area of the "piston." Assuming an equal dart mass, an increase in pressure or "piston" surface area will result in greater acceleration as long as the pressure can be maintained.


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## NightKnight (Dec 5, 2012)

I had thought of that as well. The larger the "piston" in a pnumatic, or hydraulic, ram the more power it has. But, I wondered if that really mattered when there is very little resistance.


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## neondog (Dec 29, 2012)

Even when you are just punching paper targets, if there is a cross wind, heavier darts can afford an advantage. If the darts are too heavy however you wind up lobbing them in and have to hold well above the target. This is due to a noticeable amount of resistance.


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## NightKnight (Dec 5, 2012)

I found a site where a guy says that he got a 22% increase in speed when using a 7' blowgun over a 4'. He was using a 20" long hunting dart, and he was able to get 185FPS with the 7' and 145FPS with the 4'.

https://sites.google.com/site/blowgunhunter/Home/faq-s


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## craftsman (Sep 24, 2014)

Night, correct ... but an assumption is being made in your calculations, that you can properly adjust the amount of air shot through the different caliber barrels, so that the exit velocity is identical. Being able to control one's breath from one shot to another in the same caliber is near impossible.

You're also forgetting to calculate for the difference in weight of the various calibers (generally, 0.40 is 0.8 gm, 0.50 is 1.0 gm, and o.625 runs about 3 gm) - now add to that, the internal diameter, for friction to the tail cone, adding more drag as the suface area of the tailcone touching the barrel increases with the caliber. It is inherently easier to shoot an 0.40 cal - but due to the extreme light weight of the dart, in flicht, it may actually be slower thatn the 0.50. Likewise, the weight of the 0.625 will slow it down.

The internal ballistics of the increased barrel length are well documented in Dr. Marinas' book (Pananandata: Guide to Sport Blowguns), and yes, there can be an increase in dart speed, but it also takes more air to get the dart out over the increased length. The increased length does not necessarily translate to a faster muzzle velocity, but rather an increased accuracy. Thus the penalty imposed in competitive shooting for barrels exceed the standard length.


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