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blow-forward pellet gun

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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blow-forward pellet gun

Unread postAuthor: jackssmirkingrevenge » Fri Aug 17, 2007 11:14 am

I've decided to take a break from developing preloaded cartridges and am currently focussing on something that's been my holy grail for years now, a pneumatic machinegun that goes "dakka dakka dakka!" as opposed to the water-hose "braaaap!" style of vortex/cloud BBMGs.

At first I pondered this relatively simple idea, based on the concept I used in my cartridge fed pneumatic, the same idea but eliminating the cartridge, the concept looked something like this:

Image

After looking at this design however, I thought I'd make something similar scaled down 6mm calibre, currently making a prototype (photos to follow as construction proceeds). i'm building it as per the animation attached below. The hope is that with a constant flow of air, the bolt will continue to cycle to give fully automatic fire. Ideally, one would be able to fine tune spring strength in order to achieve this, but once the prototype is finished it will be all sealed up, so I'm hoping to be able to vary the flow of air into the system to tweak it. If not, yet I still have it functioning for one shot per burst of air, I suppose I'll just have to find a way to "pulse" the air from my chamber.
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Unread postAuthor: mopherman » Fri Aug 17, 2007 11:19 am

very very cool.. on the first design you could scrap the spring and add a handle to the peg for semi auto.
btw, what do you use for airsoft barrels?
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Unread postAuthor: spud yeti » Fri Aug 17, 2007 11:26 am

I really like this concept. I think you should leave it auto personally. But great looking design :)
What ROF are you aiming for with this? And how much epoxy for construction :D

EDIT: I can only see one issue though. What is going to allow the spring to reseat if the airflow is constant? Is the pressure drop enough to allow for this?
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Last edited by spud yeti on Fri Aug 17, 2007 11:32 am, edited 1 time in total.
really good quote/phrase here
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 17, 2007 11:26 am

mopherman wrote:very very cool.. on the first design you could scrap the spring and add a handle to the peg for semi auto.


That would work but you'd need a 100% airtight seal when the bolt is closed and I doubt firing would be very consistent.

mopherman wrote:btw, what do you use for airsoft barrels?


Many R/C hobby shops stock materials such as brass and aluminum tube in 36" lengths - have a look here for example. Not sure what sort of dimensions are available in the US but at my local supplier I get an exact 6mm inner diameter tube :)

What ROF are you aiming for with this? And how much epoxy for construction


No idea what the rof is going to be like but I'd like something like 2-3 rounds per second - hopefully I can tweak it by playing with the airflow.

Epoxy usage so far is around 9cm<sup>3</sup>, I've attached the bolt housing to the barrel and the bolt is curing, tomorrow morning I'll remove and finish it then all I have to do is seal up the back, so hopefully I'll have some test results by Monday.
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Unread postAuthor: spud yeti » Fri Aug 17, 2007 11:33 am

note my edit please :wink:
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really good quote/phrase here
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 17, 2007 11:36 am

The hope is that the bolt will gain sufficient momentum to overcome the air pressure and continue to move back enough propelled by the spring giving it time to chamber another projectile before starting to move forward again.
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Unread postAuthor: Hotwired » Fri Aug 17, 2007 11:36 am

I think the second one has a better action and would be easier to achieve seals on.

Not too sure about how a seal could be guaranteed if the air port is from the side onto the bolt - it would also give unwanted upward force on the bolt methinks.
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Unread postAuthor: hi » Fri Aug 17, 2007 11:36 am

seems simple enough. i do have one question though.the thing that looks like a guide rod, how air tight does that have to be?

it seems like it would have to have more or less the same operating pressure when firing because if there isnt enough pressure the piston wont go foward, but if there is to much it wont go back. am i right or am i thinking about someting that doesnt really matter?
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 17, 2007 11:44 am

Hotwired wrote:Not too sure about how a seal could be guaranteed if the air port is from the side onto the bolt - it would also give unwanted upward force on the bolt methinks.


The pressure on the bolt face is consistent even if the air feed is off centre, i don't see why it should place any asymetric stresses on the bolt.

i do have one question though.the thing that looks like a guide rod, how air tight does that have to be?


I'm not using any o-rings here, relying purely on tight fits and crossing my fingers :)

it seems like it would have to have more or less the same operating pressure when firing because if there isnt enough pressure the piston wont go foward, but if there is to much it wont go back. am i right or am i thinking about someting that doesnt really matter?


The key here is momentum - even then the air starts to flow through the barrel, I'm hoping the bolt will not start to travel back immediately giving the pellet time to leave the barrel.
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Unread postAuthor: Hotwired » Fri Aug 17, 2007 11:46 am

jackssmirkingrevenge wrote:
Hotwired wrote:Not too sure about how a seal could be guaranteed if the air port is from the side onto the bolt - it would also give unwanted upward force on the bolt methinks.


The pressure on the bolt face is consistent even if the air feed is off centre, i don't see why it should place any asymetric stresses on the bolt.


I meant the first design where the air feed is from the side of the bolt.
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 17, 2007 11:50 am

Hotwired wrote:I meant the first design where the air feed is from the side of the bolt.


In that case you have a point - though in practice, the idea would have been to have the porting all around the bolt tube, using components that were tight fitting but not 100% airtight (on the grounds that a little wastage can be afforded).
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Unread postAuthor: spud yeti » Fri Aug 17, 2007 11:51 am

I can only see one issue though. What is going to allow the spring to reseat if the airflow is constant? Is the pressure drop enough to allow for this?
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really good quote/phrase here
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Unread postAuthor: jackssmirkingrevenge » Fri Aug 17, 2007 12:00 pm

spud yeti wrote:Is the pressure drop enough to allow for this?


That's the hope - to be honest I haven't done much calculation for this one, it's more trial and error, I'll only know when it's tested.

The plan is to hook it up to a compressor directly, and I can alter the flow using the screw valve on the compressor.
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Unread postAuthor: spud yeti » Fri Aug 17, 2007 12:16 pm

sounds like a plan to me :wink:
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really good quote/phrase here
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Unread postAuthor: Acdcmonkey1991 » Fri Aug 17, 2007 12:34 pm

Wow, no way, I had a design almost exactly like this the other day. The only difference is that mine has a trigger mechanism.
The trigger is a clothespin with a metal rod going through it so when I pull the trigger, the clothespin opens and lets the piston pass, which dumps air into the barrel. The spring will be set to give at 100 psi, so if I hold down the trigger the cycle repeats itself. The yellow bands are O-rings.
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