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Another Full Auto BBMG idea and prototype

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Another Full Auto BBMG idea and prototype

Unread postAuthor: punkin » Sun Dec 16, 2007 10:06 pm

I've been lurking for sometime, and finally joined so I can solicit feeback on a few ideas.

I've follow JSR's trials and tribulations of trying to create a blowback or blowforward bolt design with much interest, as I too desire to build a high velocity / low cyclical rate BBMG.

After having zero success attempting to duplicate the Swanstrom receiver design, and much reading, I am currently prototyping according to the attached diagram.

The design is quite similar to what JSR is working on, just kind-of another way of doing the same thing. The design is not mine, I found it on the BBMG forums online.

The reason I chose this design is that I am prototyping in HDPE, and I can play around with the location of the transfer tube to (hopefully) achieve the best results. If I try one location and don't like it, I can insert a 1/8" npt plug and redrill the "receiver" for another location.

What I learned today:

1.) For this design (or possibly the 20 degrees F that my shop was today), epoxy pistons are too brittle. After nearly achieving full auto cycling with constant flow of air, I broke two different pistons (likely due to slamming into the back of the receiver on recycling).

2.) A steel piston with O-ring is possible without machine tools -- I cut a 3/8" X 1" piston out of a plate of steel with a hole saw, then chucked it up in my drill press and used a small file to create a groove for the o-ring.

3.) It looks like the combination of the heavy steel piston and a rather long (4-5 inches) bolt, and proper placement of the transfer tube inlet and outlet I will be able to slow the rate of fire to a reasonable rate that will allow a gravity fed bb magazine.

I don't have any pictures today, pending whether tomorrows tests are positive or whether the prototype ends up in the trash, I'll take some photos. If I have REAL success, I'll post a short video.

Thanks guys (JSR, Clide, and others) for all the great ideas here!

--Scott
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Unread postAuthor: ShowNoMercy » Sun Dec 16, 2007 10:13 pm

Why not just make it like Clides design and just have a blow through bolt? That would take out the bends.
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Unread postAuthor: octane89 » Sun Dec 16, 2007 11:41 pm

And that way the air could flow much easier. Objective with air flow is make it easy for it to go from point A to B. I dont see a point in the magnet either.
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Unread postAuthor: punkin » Mon Dec 17, 2007 12:09 am

No disagreement here...I've been emailing with JSR, and I wholeheartedly agree that the blowthrough bolt makes more sense--for several reasons, including the fact that (pending bolt size), you can theoretically start the projectile forward with the bolt, and the blast of air (depending on chamber design) will be more effective than the design shown.

However, I believe JSR's earlier abandoned work has promise, and simply needs tuned to slow down the cyclical action.

The only reason for the "clunky" transfer tube at THIS point is for simple prototyping and testing. The final design would incorporate the tranfer tube machined into the aluminum chamber/receiver assembly.

Finally, the honest answer is--I saw this design, and had the materials to play with it--I've got to go buy stuff to play with blow through bolt designs.

Oh...and the magnet? Seems to make sense to me--it's to keep the bb from rolling out of the barrel if it's held less than parallel to the ground!

--Scott
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 17, 2007 12:57 am

Good to see you here Scott :) My own "blow-through" prototype is curing, here are the bits:

Image

Hopefully it will be ready for testing when I get back from work. looking forward to seeing your own results, do share them regardless whether it's a success or failure as one can always learn fro mmistakes :)
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Unread postAuthor: ammosmoke » Mon Dec 17, 2007 1:07 am

always learn fro mmistakes :)


LOL, I like how JSR made a mistake in posting this.... :lol:
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 17, 2007 1:32 am

I wish it was subtle irony, but in truth it was an I-should-be-on-my-way-to-work post :p
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Unread postAuthor: Eddbot » Mon Dec 17, 2007 1:51 am

what, do you work for half an hour then go home? or are you using the work computers to get on here? tsk, tsk
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 17, 2007 1:59 am

or are you using the work computers to get on here?


*innocent whistle*
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Unread postAuthor: Eddbot » Mon Dec 17, 2007 2:27 am

if the pressures coming from a blow gun make the bolt move too fast why don't you tone just down the pressure? wouldn't that make it cycle slower?

JSR: i just realized you live in EU, that would explain why you're going to work at 11 o'clock at night
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 17, 2007 2:31 am

if the pressures coming from a blow gun make the bolt move too fast why don't you tone just down the pressure? wouldn't that make it cycle slower?


I had tried varying the flow but i only got two outcomes, either it cycled too fast or didn't cycle at all. With the above prototype I'm hoping I can tune it by varying the strength of the bolt return spring.
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Unread postAuthor: Eddbot » Mon Dec 17, 2007 2:34 am

maybe when you brought the flow down you left the stronger spring on?
weak/er flow + weak/er spring = manageable cycling
oh darn i skipped the 2nd half of your post :oops: i'm restating old stuff, im tired :coffee:
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Unread postAuthor: POLAND_SPUD » Mon Dec 17, 2007 2:53 am

LOL recently there are lots of designs of semiauto/fullauto guns here... and i like it :D I hope that after christmas at least one person would show as a finished project...
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Unread postAuthor: Antonio » Mon Dec 17, 2007 3:16 am

Hey man I like the design of the airgun. The thing is that u have to work with constant airpressure. I have been thinking of building a regulator for my airguns. I havent seen any homemade regulators yet on this site. But yeah if everything is balanced out>spring, piston and air pressure it should work fine. I think I have seen this design before though somewhere on the net. You know whats funny, all the good guns that are made here are really similar to paintball designs> I like that. Good luck building! Could you plz give a comment on my airsoft guns as well?
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 17, 2007 3:21 am

POLAND_SPUD wrote:LOL recently there are lots of designs of semiauto/fullauto guns here


I think people are finally realising that for a give pressure, vortex and cloud BBMGs have reached their performance limits, as well as being tired of having to pick up all those BBs :D

Looking at my design, punkin made a vald point about the need for a heavy bolt - I might need to add some lead to mine as it's quite a featherweight.

For this design (or possibly the 20 degrees F that my shop was today), epoxy pistons are too brittle. After nearly achieving full auto cycling with constant flow of air, I broke two different pistons (likely due to slamming into the back of the receiver on recycling).


I think it would be a good idea to incorporate rubber bumpers at the areas where moving parts make contact in order to mitigate the stress on the system.
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