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Need Some Help With a Blow-Forward Semi

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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Need Some Help With a Blow-Forward Semi

Unread postAuthor: noname » Mon Aug 03, 2009 11:47 pm

I'm planning on making myself an airsoft rifle and have just about everything figured out except the breech. I plan on using a larger fill chamber connected to a smaller firing chamber via ball valve and regulator, with a 1/4" QEV as the firing valve. I'll be using 7/32" brakeline sleeved in 5/16" brakeline as a barrel.
Since it's something I'll actually want to shoot a lot I don't want it to be a muzzle loader, and I don't want to deal with a blow forward bolt because I have before and those are annoying as hell, I've decided to go with breech loading. The coolest way being bolt action of course. There's only two things that I'm having trouble with: figuring out how to stop the BB from rolling down, and how to attach a small handle like a screw to 1/4" brakeline without seriously blocking air flow.
If any of you guys have any solutions I would be very pleased, and thank you in advance.
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Last edited by noname on Thu Aug 06, 2009 10:12 pm, edited 1 time in total.
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Unread postAuthor: hi » Tue Aug 04, 2009 12:37 am

to stop it from rolling back, drill a small hole, as small as you can, and use the smallest nail you can that fits and put it behind the breech. cut it to size and use either a drop of glue or just wrap tape around it to hold it still. to keep it from rolling forward, use a little bit of cotton or cloth. thats how i did it and it worked well.

as for the handle maybe use hose clamps to clamp on a block of wood, then drill a hole to screw in the screw.


sometimes the best way is the simplest way.
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Unread postAuthor: MrCrowley » Tue Aug 04, 2009 12:41 am

To stop it rolling forward you could try making a very small indent in the barrel with a nail punch (because it has a more surface area than a nail or screw which might damage your barrel too much)
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Unread postAuthor: noname » Tue Aug 04, 2009 11:58 am

I'm ditching the bolt action idea. Looking through my parts box I found the remnants of my semi auto blow forward bolt so I'm going to salvage as much of that as I can. I need to make my own custom T which will be annoying, then figure out a good gravity fed magazine and stick it all together with a barrel. Lots of fun.
I also remembered a way to stop a BB the worked well before, cut a notch in the top of the barrel and put an oring around the barrel so that some is in the barrel but a BB can still squeeze by.
And anyone know if <a href="http://airinc.thomasnet.com/viewitems/specialty-valves-acc-/super-quick-exhaust-valves?&forward=1">these QEVs</a> are any good?
Any suggestions are very welcome.
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Here's a diagram of the blow forward bolt so far
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Unread postAuthor: Scottaiii » Wed Aug 05, 2009 7:08 am

What i used to do to stop it from rolling out the barrel was cut the end of a water ballon (The Stem) And tape the cut end to the outside and insert the end with the ring on the inside. This shouldn't stop the bb, as it should just build the pressure up behind it, and cause it to fire.

Combine that with say a small ball valve with a hole drilled in it, simple and it works.

- Scott.
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Unread postAuthor: inonickname » Wed Aug 05, 2009 7:19 am

If you cut the barrel with a tubing cutter, and clean up the burr the ammo won't come out, but there will be little flow restriction.
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Unread postAuthor: noname » Thu Aug 06, 2009 10:11 pm

I modified the blow forward bolt a bit and set up a gravity feed system. It all works when hooked up to a blowgun and air compressor; I'd like to see the difference in performance with propane, a firing chamber, and the 1/4" QEV I ordered today.
The blow forward doesn't work perfectly, it stops and returns to the seat without traveling forward all the way and leaving about 1.5 mm of room for air to get out and blow BBs up out of the temporary magazine. It usually blows forward all the way with no spring, but won't return to the seat if there isn't a spring. If the string is too strong then it travels even less, which results in random 2 or 3 shot bursts.
Does anyone know how to fix the burst problem, while still having the bolt blow forward and re-seat itself?
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Unread postAuthor: jackssmirkingrevenge » Thu Aug 06, 2009 10:17 pm

How easy is it for the bolt to move? It sounds like you have too much friction in the system, ideally everything is as smooth as possibe - even if this means making it slightly loose - so you can get away with using as weak a spring as possible.
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Unread postAuthor: noname » Thu Aug 06, 2009 10:25 pm

Well see the really annoying thing is that I had to take 1/2" off of the bolt because it got bent. Which means I also had to take 1/2" off the tube the bolt rides in. So the bolt travels through a tiny bit of 1" PVC and then some 1" female adapter threads. It slides fine but that might be causing the problems.
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Unread postAuthor: noname » Fri Aug 07, 2009 12:31 pm

As of today I'm also ditching the blow forward bolt remains. In exchange, I think I might just make a blow back bolt. It seems like there are less possible problems with a new blow back versus a frankenstein blow forward.
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Unread postAuthor: POLAND_SPUD » Sat Aug 08, 2009 12:36 am

you might consider trying the mechanism I used on my semi auto
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Unread postAuthor: noname » Sat Aug 08, 2009 12:56 am

If you have a link that would be greatly appreciated; 12 minutes of searching got me nowhere.
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Unread postAuthor: POLAND_SPUD » Sat Aug 08, 2009 1:09 am

be warned that you'd need to buy a 3 way valve and an air cylinder to build it

http://www.spudfiles.com/forums/i-has-s ... 16872.html
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Unread postAuthor: noname » Sat Aug 08, 2009 1:14 am

Thanks but I'm pretty sure I'll be going with my blow back. I spent today constructing it and probably around an hour screwing around with it and trying to get it to work. The bolt hasn't been blowing back but air still escapes out the barrel and magazine. So I sealed it up (I'm pretty sure) and I'll try again tomorrow.
My design is fairly simple, I'll draw it up in paint right now.
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Unread postAuthor: POLAND_SPUD » Sat Aug 08, 2009 1:23 am

edit

ohh I see... I can't think of any reason for it not to work... but that doesn't mean anything really :cry: most problems surface when you go from designing to building

edit no 2
upps :shock:
I just got a closer look at the pic....it won't work... you need to make sure that mag is blocked off when air passes...

you'd need a small port cut in the tube to let ammo in (the bolt needs to have the same ID as the barrel so it has to slide inside a pipe larger than the barrel

I hope you cna understand what I mean??
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Last edited by POLAND_SPUD on Sat Aug 08, 2009 1:52 am, edited 2 times in total.
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