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Brass barrel help

1" mongo darts with an airblaster?

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#1 Gin

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Posted 17 April 2015 - 10:24 PM

how long does my brass need to be to maximize preformance with mongo darts from an airblaster? I don't know if I am just incompetent in searches or what but i can't find what length I will need
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#2 Birch

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Posted 18 April 2015 - 11:09 AM

Mongo darts are 3/4 inch right? i would be surprised if you can find brass that big. In any case, calculate the total volume of the barrel you would use for micros and cut your mongo barrel to a length that will have the same value as the micro one. Of course, it all comes down to testing and trial and error.
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#3 Gin

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Posted 18 April 2015 - 01:29 PM

Mongo darts are 3/4 inch right? i would be surprised if you can find brass that big. In any case, calculate the total volume of the barrel you would use for micros and cut your mongo barrel to a length that will have the same value as the micro one. Of course, it all comes down to testing and trial and error.


1 inch and thank you for the help. I have a place selling 1" brass around here but they need to order more as all they have right now are 20 foot chunks and they will not cut them down lol. so they said if I let them know how long I need it they will order that length for me.

now I just need to figure out the value for micro >.< ah well you still made things much easier for me lol.
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#4 Draconis

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Posted 18 April 2015 - 03:52 PM

Don't bother with brass, that is silly and ridiculously expensive. Most 1" Sch 40 PVC has a good inside diameter for Mongos.
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#5 Cartaya

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Posted 18 April 2015 - 07:15 PM

OK, try this:


Volume of tank X Atmospheres - tank /Barrel internal area= 1.7

Example:

30cc's X 3(45psi)-30cc's = 60/11"cpvc (3.211cc's per inch)=60/35.321 =1.698 = 25.48 PSI at dart leaving barrel.

You need at least 1.7 ATM to get good power, maybe more like 2.0 or 2.2 for a 1" dart, given it will have more friction.
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#6 OzNerf-Sarge

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Posted 20 April 2015 - 06:39 PM

OK, try this:


Volume of tank X Atmospheres - tank /Barrel internal area= 1.7

Example:

30cc's X 3(45psi)-30cc's = 60/11"cpvc (3.211cc's per inch)=60/35.321 =1.698 = 25.48 PSI at dart leaving barrel.

You need at least 1.7 ATM to get good power, maybe more like 2.0 or 2.2 for a 1" dart, given it will have more friction.

Want an equation for ideal barrel length for air guns? Look below!

Posted Image

This equation applies for md > 100 * rhoatm * Vc (approximately). The heavier the dart, the more accurate this equation is (technically this equation applies for md = infinity). md could be lower than that value and this equation could still apply. Check the tables to be certain.

All pressures are absolute.

Lb is barrel length
Vc is chamber volume
Pc is chamber pressure
Pf is "pressure of friction"
k is the ratio of specific heats (1.4 for air)
Vd is "dead volume"
Ab is the cross sectional area of the barrel (i.e. pi * d^2 / 4)
md is dart mass
rhoatm is atmospheric air density

This equation matches my numerical results nearly perfectly. While I did not derive it from the ODEs used for my simulation, I believe it is accurate and correct.

I'm going to work on the springer tables and equation now...

Edit: By similar logic, the equation below should apply for springers.

Posted Image

Vc is the volume of the plunger tube up to the plunger head (I should use a different variable name)

I haven't checked the spring equation against simulations or empirical data, so take it for what it's worth.


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#7 Gin

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Posted 21 April 2015 - 06:52 AM

Don't bother with brass, that is silly and ridiculously expensive. Most 1" Sch 40 PVC has a good inside diameter for Mongos.


what about the friction though? I mean I will probably go with the pvc as you have helped me quite a bit in the past so I trust you when it comes to these things, just wondering what the difference is?
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#8 ompa

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Posted 21 April 2015 - 08:25 AM

Provided you just deburr the pvc pipe (if you used a saw of sorts) or make sure the inner lip is flush with the rest of the inner barrel (if you use something like a pipe cutter), you shouldn't run into any issues. And it would definitely be way cheaper.

Unfortunately I can't really give you the actual coefficients of friction for pvc and brass, but I can tell you that it has been used with great success at least when I was more active.

Edited by ompa, 21 April 2015 - 08:26 AM.

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