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1.5" vs. 2"

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: PAspuder » Thu Apr 12, 2007 3:09 pm

I think that potatos go alot better out of a 1 1/2" because the smaller barrel allows for more force to push it out. For other objects, 2" offers more projectiles. I use the 1 1/2" 9 times out of 10, and i have 5 barrels all together.
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Unread postAuthor: SpudBlaster15 » Thu Apr 12, 2007 3:31 pm

jimmy101 wrote:I thought the affect of air drag was dependent on the sectional density of the round. The sectional density of a 1.5" and 2" diameter round should be the same. They should decelerate such that their kineitc energies drop off at the same rate.


Yes, you are correct. For some reason, I forgot that the mass of the rounds differs.

jimmy101 wrote:Their ranges, at the same launch angle, should be very similar.


If the velocities are the same, yes. If the energies are the same, no. That would mean that the 1.5" round is traveling at a higher velocity. By the time it has slowed down to the velocity of the 2" round, it will have gone farther.
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Unread postAuthor: jimmy101 » Thu Apr 12, 2007 3:36 pm

judgment_arms wrote:
jimmy101 wrote:...Arne't most rifle rounds designed for extreme ranges fairly large? Is it possble to build a 0.22 that has an affective range of a couple thousand yards?


Not unless the round was made out of something denser then lead, it’s all about mass, the heavier the projectile the farther it will go. Wad up a 3X3 inch piece of paper and throw it then wad up a piece of aluminum foil the same size and throw it. Which goes farther?

Also with the 2.00cal barrel you’ll have a higher flow coefficient. Fire a 1oz projectile out of a 1.50cal barrel and then fire a 1oz projectile out of a 2.00cal (run a simulation in the GGDT only changing barrel diameter.) the 2.00cal barrel will yield a high velocity.


Exactly (well sort of).

You really can't compare the same mass round from two different diameter barrels (well you can, but it doesn't mean much).

A better comparison is a 2oz projectile from a 2" barrel versus a (2oz)(1.5<sup>2</sup>/2<sup>2</sup>)=1.125oz. We are talking about the way a typical spud is cut in this size barrel. The smaller barrel gives a smaller diameter, hence lighter, spud.

goes and fires up ggdt...

All defaults except spud's mass and diameter, barrel diameter and barrel length set to 150". Spud mass scaled by the ratio of the squares of the barrel ID.

2" barrel, 100g spud ==> 470fps, KE=1022 J
1.5" barrel, 56.2g spud ==> 600fps, KE=941 J

Big difference in muzzle velocities. (Doesn't match my "same acceleration" statement.)
Difference in kinetic energy only ~8% (The larger barrel should be calculated using a larger value for friction, but I didn't bother, this would lower the KE of the bigger barrel a bit.)
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Unread postAuthor: boilingleadbath » Thu Apr 12, 2007 5:47 pm

The way I see things is that, given the same sectional density (as jimmy says), the smaller diameter barrel will produce a higher velocity for two reasons:
*The amount of air 'used' is smaller, so the pressure drop is smaller, so the average pressure is higher.
*The pressure drop across the valve is smaller with a smaller barrel due to a lower rate of air 'consumption'.

However, in must scenarios, the larger diameter barrel will give you more muzzle energy, and possibly more range due to a higher sectional density when it's tumbled sideways.

How this translates into damage is going to depend heavily on what you are shooting.
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Unread postAuthor: frankrede » Thu Apr 12, 2007 6:38 pm

I like 1.5" barrels more.
But I have boughten potatoes large enough so that 2 shots per potato in a 2" barrel.
Those at ht Vegas meet saw my mutated taters lol/
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Unread postAuthor: Solar » Thu Apr 12, 2007 7:33 pm

Holy ballistics! I was just curious about how many people were using what type of round. I tried setting up a poll, but it didn't work for some reason. Some great info however. Thanks for all the responses.
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Unread postAuthor: Orpackrat » Thu Apr 12, 2007 8:00 pm

I prefer 2" barrels over 1.5" barrels because my homemade paintball grenades fit better in the 2".
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Unread postAuthor: jimmy101 » Fri Apr 13, 2007 10:16 am

Boilingleadbath:

You are right of course. I assumed that the pressure was the same for both barrels (the always tricky "all other things being equal" approach).

The pressure will drop faster with the big barrel so the acceleration will be less with the big barrel than the little one.

And the drag through the valve will be worse for the larger barrel since more air has to be moved.

I went through the full calculation last night. The acceration of the spud will be the same in the two barrels if the following conditions are true;

1. The spuds are the same length (which is probably true).
2. The friction of the spuds is small. The larger diamter barrel will have more friction (scaling linearly with r) but if the friction is low enough it doesn't really affect the acceleration much. The default friction in GGDT is just 3 PSI, not much compared with say a 120 PSI chamber pressure.
3. The volume of the chamber is much larger than the volume of the barrel. The larger barrel uses more air but if the chamber is larger enough the difference is small.
4. The throat of the valve is large compared to the diameter of the barrels so that the different volumes of air that have to move through it experience only minimal drag.
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Unread postAuthor: jimmy101 » Fri Apr 13, 2007 10:34 am

Spudblaster:
Didn't you post a message with the mass of a spud cut in 2" versus 1.5" barrels? It also included an estimate of the sectional densities for the two rounds.
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Unread postAuthor: SpudBlaster15 » Fri Apr 13, 2007 12:33 pm

I deleted it because I realized that the batteries in my scale were on the verge of death, and I thought the weight readings would be inaccurate. Today, I found new batteries, and re-measured the 6 cut slugs. The average of the 1.5" slugs was once again 55 grams, and the average of the 2" slugs was once again 100 grams. The length of each slug was identical.

I suppose the condition of the batteries (unless entirely depleted) does not have an effect on the readings.

So the sectional densities are...

2.049/100 = 0.02049

1.6/55 = 0.02909

Since I am not sure how this difference translates to the actual drag force exerted on the spud, I ask you, is it significant?
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Unread postAuthor: boilingleadbath » Fri Apr 13, 2007 1:50 pm

Eh, spudblaster, sectional density is mass/area, so the calculations would be:

100/2.049<sup>2</sup> = 23.8 g/in<sup>2</sup>
55/1.6<sup>2</sup> = 21.5 g/in<sup>2</sup>
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Unread postAuthor: SpudBlaster15 » Fri Apr 13, 2007 2:56 pm

Yeah, oops. I need to again start thinking before posting.

Wouldn't the calculation be M/(pi*r^2), since the spud is a cylinder?

100/(pi*1.025^2) = 30.3 g/in^2

55/(pi*0.8^2) = 27.4 g/in^2

(How do you get the exponents to show up in your post?)
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Unread postAuthor: boilingleadbath » Fri Apr 13, 2007 3:23 pm

Ah Fark... yeah, it's cylindrical.

Anyway, exponents are done using the superscript tag:
blah(sup)blah(/sup)

(but the paranthases are angle brackets.)
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Unread postAuthor: jimmy101 » Sun Apr 15, 2007 11:37 am

So the difference in sectional density is about 10% and the spud mass is roughly proportional to the barrel diameter squared, not cubed.

I don't think a 10% difference it terribly significant, only a modest change in how well the spud will carry.

What was the standard deviation of the weights of the sets of three slugs? It is possible that the difference you measured is just from the variability of the individual slugs.
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Unread postAuthor: SpudBlaster15 » Sun Apr 15, 2007 1:03 pm

I tried to make them as close as possible.

1.5" slugs:

#1: 54.3 grams
#2: 56.1 grams
#3: 55.4 grams

I read these as 54, 56, and 55 because my scale is accurate down to only 1/10 of a gram.

2" slugs:

#1: 102.1 grams
#2: 98.9 grams
#3: 100.2 grams

I read these as 102, 99, and 100. The average is slightly over 100, but I used 100.

I did notice that the 1.5" slugs flew farther after leaving the barrel of my launcher, however. :wink: (Not by much, a couple dozen yards at most, I didnt have a tape measure.)
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