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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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Unread postAuthor: noname » Sat Aug 08, 2009 1:32 am

Yeah, unfortunately if the bolt is a tight enough fit with the barrel it doesn't want to move. But if it is loose then the air will just leak out the barrel instead of actuating the bolt. This whole thing is just really annoying. I would just say screw it and settle for full auto but I really want to make a decent semi this time.
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Unread postAuthor: POLAND_SPUD » Sat Aug 08, 2009 1:56 am

see my reply on the previous page

the pic shows how it can be done...
I suspect that there are two possible ways to do it... one of them is the one presented in the pic (note that the bolt has to move prior to the firing)

the other option is similar to the one in your pic but you'd have to make sure that the mag is blocked off when the gun fires...
you could either build it in a way that won't require the bolt to move to let air pass through or one where the bolt would have to retract slightly to pass air...
the latter would work better IMO as there would be more force that would help to overcome inertia of the bolt... with these designs ammo would be loaded after firing not prior to it

obviously with this mechanism you would have to either dry fire teh gun first to load ammo or build some sort of handle to load the first round before firing
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 08, 2009 2:02 am

Personally I think the blow forward bolt makes more sense, it probably takes less effort to operate and wastes much less air.

This is the blowback design I had come up with:

Image

Never actually made it though, it was an improvement on this design which wouldn't work reliably at compressor pressures due to the small surcafe area the bolt had to push on.
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Unread postAuthor: noname » Sat Aug 08, 2009 2:07 am

I could probably make those adjustments without too much trouble. I'm using 3 different sizes of tubing. The bolt is the smallest, 1/4", the tube it rides in is the largest, 5/16", and the barrel is in between, 9/32". The 9/32" tubing fits tightly over the 1/4" tubing so I can add on with a larger diameter and still have it ride inside the original 5/16" "T" tube. I just hope there isn't too much friction between the 9/32" tubing and the 5/16" tubing. They're both aluminum, so it shouldn't be too much I hope. They do fit very tightly though.
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 08, 2009 2:14 am

Have you considered a simple tee magazine like this or this?

It worked perfectly well for me ;)
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Unread postAuthor: noname » Sat Aug 08, 2009 2:10 pm

Jack, those would both be extremely easy to make, but I would either get full auto or bursts instead of semi auto.
Poland, I'm going to add on to the bolt as shown in the picture and do some testing to see if it works. Results in a few hours.
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 08, 2009 2:15 pm

noname wrote:Jack, those would both be extremely easy to make, but I would either get full auto or bursts instead of semi auto.


I got full auto because the valve was reciprocating, as you can see every blast of air fires only one projectile. The idea is that pressure pushes one projectile down the barrel and pushes the rest up the magazine. Unless your valve stays open for a long time, you'll only get one shot per valve opening.
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Unread postAuthor: noname » Sat Aug 08, 2009 4:28 pm

That sort of makes sense so I guess I'll try it if nothing else works.
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Unread postAuthor: ballores » Sat Aug 08, 2009 4:42 pm

could you use a strong magnet to keep the steel bb in place also guys wondered on the legality of co2 cannons over in the uk cause i have a good one cheers Nick
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Unread postAuthor: jackssmirkingrevenge » Sun Aug 09, 2009 2:50 am

ballores wrote:also guys wondered on the legality of co2 cannons over in the uk cause i have a good one cheers Nick


Technicallt iy it's capable of going over 12 ft/lbs of energy or if it's full auto with more than 1 ft/lb per shot it needs to be licensed as a firearm. Technically ;)
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Unread postAuthor: POLAND_SPUD » Sun Aug 09, 2009 5:05 am

I have to agree with JSR here that this is one of the best designs you can choose...
the tests I conducted long long time ago were rather dissatisfying but now I realize that was mostly a result of hasty construction and lack of proper materials

the design has some serious potential because it is as simple as it can be
give it a try...
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Unread postAuthor: jackssmirkingrevenge » Sun Aug 09, 2009 5:15 am

POLAND_SPUD wrote:Now I realise that was mostly a result of hasty construction and lack of proper materials


The critical parameter is a tight fitting magazine tube, otherwise all the BBs will get pushed out throught the barrel at once as you've effectively created a cloud BBMG.
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Unread postAuthor: noname » Sun Aug 09, 2009 12:13 pm

Yesterday I tried dry firing the blowback bolt without anything glued or bolted together and of course the bolt stuck. Which meant the whole tube/chamber that the bolt rides in got blasted off straight into my thumbnail, ramming probably a millimeter backwards into my thumb. I now have a nice purple bruise around my cuticle and underneath my nail.
So I got pissed off at it and decided to go back to a blow forward bolt :D I need to go to the hardware store and pick up 2 PVC parts and then I'll be able to test it because I remade everything yesterday and I think it will actually work this time.
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Unread postAuthor: noname » Tue Aug 18, 2009 11:20 pm

OK, update as of now. The gun is put together. Not complete, but put together. I did attach a basic single shot barrel so that I could do some test shooting to get general velocities.
Then I started by pulling out my two airsoft spring pistols and shooting cans with them at point blank in the exact same spot each time. The guns are said to shoot in the 300-320 fps range. Each one put a shallow dent in the cans.
Next, I hooked up the propane tank to the gun and set the regulator to 100 psi. I was hoping that this shot would be much too strong to use while playing with friends.
100 psi blasted the BB straight through both sides of the can. The same shot against skin would be horrible.
75 psi put the BB through one side. Not too bad but still too strong.
55 dented the can deeply.
40-45 put a dent in the can about the same size and depth as the airsoft pistol shots. This will be my working pressure while playing with friends.

The blow forward bolt should be finished sometime soon. I'm hoping I can get it done but I will be really busy the next couple days. Hopefully my next post about the gun will be in the showcase area.
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 18, 2009 11:39 pm

You can use the "poor man's chrono" to get a velocity estimate using airsoft BBs and cans.
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