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making the blow-forward breech work

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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Unread postAuthor: jackssmirkingrevenge » Tue Mar 11, 2008 10:57 am

I had been playing around with the new bolt and still no success, but then I realised that the check valve wasn't working - even without the bolt fitted, air was still coming through. I've cut it up and removed the offending component, next step is to convert a schrader valve for the purpose - watch this space :)

Edit: schrader installed, chamber resealed and epoxy curing, I've also re-cast the first piston in order to have a tight-fitting lighter option in case it doesn't function with the heavyweight piston.

A small sidenote, I've found a use for old epoxy castings - and when you make so many risque prototypes as yours truly, this is a useful resource to be able to reuse. Simply fragment the cured epoxy and chuck then into the matrix when casting something new, especially a high-volume fitting like an endcap. The final product is effectively as strong as if it was a one-piece casting, and you're saving substantially on new epoxy :)
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Unread postAuthor: Skywalker » Tue Mar 11, 2008 3:24 pm

Huh, it'd be interesting if that schrader valve makes the difference! I'm surprized that the heavier bolt didn't help though. How much friction does your piston have? (I know you aren't going to measure it, just a rough idea: will the piston slide out under gravity? etc).

Don't you mean 'risky' prototypes? Or have you been making epoxy models of ... erm, anatomical appendages?
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Unread postAuthor: iknowmy3tables » Tue Mar 11, 2008 4:17 pm

I think you need to shorten rod thing sticking out from the back of the bolt, and I really don't understand the check valve, the ball seems to act like a check valve doesn't it, add a spring to behind to it should work

:!: I got an idea increase that dead space behind the bolt so it will take longer to build up and push the bolt
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Unread postAuthor: Sticky_Tape » Tue Mar 11, 2008 4:41 pm

If he increases the dead space there will definently not be much power.
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Unread postAuthor: iknowmy3tables » Tue Mar 11, 2008 4:43 pm

less power but it would slow down the ROF which i believe is his goal
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Unread postAuthor: Sticky_Tape » Tue Mar 11, 2008 4:48 pm

the rof would be ok it's not like it has the rof of a cloud And I am pretty sure he wants to keep the power.
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Unread postAuthor: iknowmy3tables » Tue Mar 11, 2008 9:32 pm

however so for my attempts have either resulted in a valve that hangs open or cycles so fast it's impossible to reload ammunition

power is great but somethings gotta give, more dead space is a quick and easy solution
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 11, 2008 11:02 pm

Skywalker wrote:Huh, it'd be interesting if that schrader valve makes the difference! I'm surprized that the heavier bolt didn't help though. How much friction does your piston have? (I know you aren't going to measure it, just a rough idea: will the piston slide out under gravity? etc).


The heavier bolt opened much slower and the muzzle blast was much less audible, I think perhaps I went a bit overboard :roll:

I'm not sure exactly when the home-made check valve failed (basically the o-ring wasn't properly secured and popped out - I should have known, but anyway), if it failed on the first shot then in principle this was functioning just like my previous design without the check valve - so there might yet be hope.

I can't really play around with spring tensions that much as that affects the release pressure of the bolt, so the only two options left are bolt weight and friction. As regards to friction, I can easily push it with one finger, and without air behind it (ie if I removed the pressure source) the spring has absolutely no problem returning the bolt to the closed position, even with the weaker of the springs that I've tried. Therefore it's not really an issue, besides reducing the diameter of the bolt enough will allow too much blowby and the bolt won't travel enough to close the breech.

Don't you mean 'risky' prototypes? Or have you been making epoxy models of ... erm, anatomical appendages?


There have been suggestions that I convert this design into some sort of female reciprocating pleasure device :roll: :D

I got an idea increase that dead space behind the bolt so it will take longer to build up and push the bolt


Rate of fire is not the issue here. If it was working as intended, then it could be altered simply by changing the flow into the chamber, easy as that. The problem here is that the bolt is opening and hanging open, from what I gather because of an ineffective check valve that allows pressure to continue leaking into the chamber even though the bolt has moved forward. Hopefull what mods I've made will address this issue, I'll find out in around 12 hours.

This is what the current setup looks like with the schrader incorporated:
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Unread postAuthor: Antonio » Wed Mar 12, 2008 1:00 pm

jackssmirkingrevenge wrote: The problem here is that the bolt is opening and hanging open, from what I gather because of an ineffective check valve that allows pressure to continue leaking into the chamber even though the bolt has moved forward. :


I think its obvious why ur valve always leaked. If you look in your schematic picture you can c the oring doesnt touch the sides of the outer tube. With my valves they do touch and thus create a perfect seal. A simple solution to your valve would have been a slightly bigger oring:) But I guess its not all that simple lol. Btw I forgot by how many times u increased the hammer/bolt but it sounded like u overdid it yeah.
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 13, 2008 12:58 am

I still can't get this thing to fire reliably, it's going to the truck of fail where it belongs :)
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Unread postAuthor: iknowmy3tables » Thu Mar 13, 2008 6:33 pm

noo don't give up, with you progress so far it must be possible,
it's gotta work maybe with more spring tensions and tighter seals, and spring bb feeding loaded feeding, its gotta work
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 13, 2008 6:41 pm

Maybe some other time in the future, but not now, it's a bit of a bad time for frustration and I want no more of it for now.
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Unread postAuthor: iknowmy3tables » Thu Mar 13, 2008 9:01 pm

well do you have something else on your mind? whats your next project gonna be
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 13, 2008 11:36 pm

I'm thinking sawn-off shotgun, still fermenting thoughts in my head though.
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Unread postAuthor: mega_swordman » Thu Mar 13, 2008 11:48 pm

jackssmirkingrevenge wrote:I'm thinking sawn-off shotgun, still fermenting thoughts in my head though.


I take it you are referring to BTB's design, or are you going for something else?
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