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First Pneumatic Gun Problems

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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First Pneumatic Gun Problems

Unread postAuthor: ntiemeier » Thu Feb 11, 2010 11:25 am

Hi Everyone,

I'm New here and new to this but since I found the Pneumatic gun design on http://www.burntlatke.com/bb.html i havn't stopped untill i had it made.

Per the instructions I modified it slighty to allow for a larger airsoft bb rather than a metal one. Basically iI created the vortex block from bondo and that went near perfect. I did not use the shims though..yet. The barrel is 1/2" aluminum tubing 36" in length. The bb's are stored in a regular 2"PVC T, pretty mutch like the picture.

Here's the issue. I finally got done and had 80lbs of pressure locked and loaded, pulled the trigger and it shod about 50 bb's constantly but only about 3 feet about the speed of a rain shower....AHHHHH!!

What am I doing wrong? Any suggestions from you all that are more experienced than I?
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Attachments
IMAG0001.jpg
Overall Gun
IMAG0002.jpg
Inside the 2" T looking from the back end.
IMAG0003.jpg
Vortex block in base and 1/2 barrel with pipe that connects to it.
IMAG0004.jpg
Vortex block
IMAG0005.jpg
Vortex Block Bottom
IMAG0006.jpg
Vortex Block Side
IMAG0007.jpg
2" T looking from Bottom where the vortex block and base connect.
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A little progress

Unread postAuthor: ntiemeier » Thu Feb 11, 2010 2:44 pm

OK, I could not wrap my mind around it for a minute. How was I able to make a medicine bottle and a bic pen case shoot faster than an 80 PSI air gun. It must be pressure. I glued a bic pen to the barrel output and as a leader into the larger aluminum barrel and I have better results. It definitely shoots faster but still need more.

I'm curious about the vortex. The air enters from the bottom of the vortex block but does it loose air pressure by allowing air to enter the t that holds the bb's?

I'm just trying to wrap my head around the physics.
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Re: A little progress

Unread postAuthor: jackssmirkingrevenge » Thu Feb 11, 2010 3:06 pm

Welcome to spudfiles, always nice to see newcomers who have done their homework :)

ntiemeier wrote:I'm curious about the vortex. The air enters from the bottom of the vortex block but does it loose air pressure by allowing air to enter the t that holds the bb's?


It does, the vortex block is a horribly inefficient design. Feeding 100 psi through a blowgun will only give you typically 20-30 psi inside the BB chamber, the reason they appear so powerful is the fact that lots of BBs are hitting the same place in quick succesion. You can appreciate it in slow motion here:


[youtube]http://www.youtube.com/watch?v=JlMJkbSrA0c[/youtube]


[youtube]http://www.youtube.com/watch?v=1gjN8P7k-p4[/youtube]


Still, you should be getting much better velocity, the above videos were made at 100 psi feeding through a blowgun. I think your barrel might be too loose, actually I'm quite sure it is - what's the difference between barrel internal diameter and BB diameter? I had done an experiment, making the barrel just 0.019" wider (0.196" for 0.177" BBs) led to a power loss of around 25%
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Unread postAuthor: ntiemeier » Thu Feb 11, 2010 3:24 pm

Interesting and makes sense. I have a 1/8" gap of free space around the BB in the barrel. Internally it only allows enough space for the BB to leave the vortex block but I can see that the air will also be coming out around the BB rather than just simply pushing it inside the barrel. I'll look for another barrel size.

Also, Do you have any suggestions of a more efficient way to set this gun up? By that I mean without using the vortex block? I've seen some that simply use 2" PVC pipe and have the air supply entering just to inside wall which creates the vortex and exits the center at the opposite end. I betting that this is the inline vortex. but would wonder how to easily feed the BBs to it. Thoughts?
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 11, 2010 3:47 pm

ntiemeier wrote:I have a 1/8" gap of free space around the BB in the barrel.


That is definitely the problem, even 1/64" is too much space.

Do you have any suggestions of a more efficient way to set this gun up?


Playing with feed/barrel positions is definitely not going to increase power. it might give you better reliability, but it won't help with efficiency.

You can improve performance by dramatically increasing input flow (say using a 1/2" ball valve instead of a blowgun) or pressure, but that doesn't make it more efficient.

One solution is to put some sort of detent (for example an o-ring seal, magnet in the case of metal BBs or a spring loaded ball detent) that will hold the BB at the breech allowing pressure to build up in the BB chamber before the BB is released. It also has the added benefit of reducing rate of fire to some extent making your design more economical with BBs.

Another solution, not as ergonomical as the blowgun but it works for power, is to have the BBs already inside a pressurised chamber and having a ball valve between the chamber and the barrel through which the BBs will flow.

Image

This means that if your input pressure is 80 psi, that BB will definitely have 80 psi behind it as it fires, as opposed to around 25% of that which a conventional blowgun fed strafer design will give you.
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Unread postAuthor: ntiemeier » Thu Feb 11, 2010 4:03 pm

Awesome, I'll try a couple different ideas here. Thanks! Great Forum!!
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 11, 2010 4:21 pm

ntiemeier wrote:Great Forum!


It is, have a browse and there are a lot of ideas here.

As to finding a better barrel, have a look round your local hobby store, many places stock brass and aluminium tubing in 3 foot lengths, in Europe you can usually find a true 6mm internal diameter barrel, might not be so close in the US but you can find the closest equivalent (take a BB along with you ;)) that will be much better that what you're currently using.
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Unread postAuthor: grock » Tue Feb 16, 2010 2:03 am

To add on to what JSR said, carbon fiber arrow shafts often work, as well as 1/4" breakline (car/truck part). You could also buy an airsoft barrel online, but the longest ones out there are only about 600mm (~2 feet). I've found the ones on this site very good, plus if you buy off this particular page, they get shipped for free. the problem with there is that they are fairly tight fitting, so you cannot use low grade or used bbs, because they will jam. (on a side note, that momentary jam might act as a detent, increasing power, who knows)
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 16, 2010 2:12 am

grock wrote:that momentary jam might act as a detent, increasing power


Better to use decent BBs and have a properly made detent you can control than rely on random ammunition defects ;)
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Unread postAuthor: Davidvaini » Tue Feb 16, 2010 2:26 am

your barrel is massively too large.. your air inlet is not even half the size of your barrel..

What size projectiles?
If its 6mm.. I'm smacking my head on my desk repeatably.

6.14mm is considered quite oversized when it comes to an airsoft barrel.. 6.25 is getting close to outrageously over sized.. half inch is 6.7mm oversized.. that means its oversized more than twice the size of your projectile...


What this means is.. the air will go the path of least resistance.. that means it will blow round the BB rather than pushing the BB down the barrel.. so all the air will go around the BB and out the barrel.. too be honest I'm surprised the BB's actually go out the barrel...


now thats just assuming you are doing 6mm airsoft BBs...

Even if you were using 8mm airsoft BBs, it is still insanely too large of a barrel...

and even if you were using 1/2" projectiles, your air inlet is too small and would lack the flow to have any decent power.

here are some helpful links:

http://www.spudfiles.com/spud_wiki/inde ... achine_Gun

and really make sure to visit this one and read it all:
http://www.spudfiles.com/forums/david-v ... 18159.html

.14mm bigger is too large.. your gap is 47 times larger than what is considered too large...

In lame mans terms:

Air goes around BB, Air does not push BB.







.
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Unread postAuthor: cannon monkey » Mon Nov 21, 2011 1:35 pm

sorry for bringing up an old thread but how do you mold the bondo to the shape i know on the (http://www.burntlatke.com/bb.html) site it says fill the 2 inch cap but wont the bondo stick to the pvc? or do i add something before i fill with bondo?
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most big random like i ever seen

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