SpudFiles

Can you guys please check if this design is sound in theory?


It's based on Antonio's design located here:
http://www.spudfiles.com/forums/viewtop ... tml#152862

I'm going for a bolt action gun, so it's not going to be automatic and there's no parts that need to be moved by air pressure.

Thanks in advance.

Looks good on paper, but some gripes:

- you're going to need fairly high pressure to make that "small burst of air" give a significant impulse to the projectile, I expressed a similar doubt in Ant's original thread. Still if it's for airsoft purposes and you don't need that much velocity it should not be an issue.

- pushing on the valve head with a plastic BB sounds like it will be putting significant strain on the projectile, make sure the tip of the head is well rounded.

Thanks jack.

Would around 100 PSI be enough?

I can always increase the length of the stem sticking up into the barrel to increase the amount of air even though more air would be used per shot.

Also, how would you recommend rounding the head?
Would a file be good enough?

Binary Blue wrote:Would around 100 PSI be enough?

For skirmishing, probably.

I can always increase the length of the stem sticking up into the barrel to increase the amount of air even though more air would be used per shot.

You can start with a long stem and shorten it to "tune" it. More power needs more air, there's no two ways about it

Also, how would you recommend rounding the head?
Would a file be good enough?

I suggest clamping a power drill in a bench vice and using it as an improvised lathe, you'll get faster and more consistent results.

How do you intend to put it together in terms of materials? I see a lot of epoxy being involved...

The air reservoir won't be attached to the barrel like my picture shows. I'm thinking of something more like this:

A note about the stem seal, if you make it like you drew it what will likely happen is the attached, the stem will move down but air pressure will keep the o-ring sealed against the port and no air will come out.

Yeah, there should be a hole around the grey stem in the green PVC end cap part. I just forgot to draw it in.

Fixed:

Better

How big is your air reservoir, and how many shots do you expect from it? 100 psi is not much for a repeater, albeit a small one.

My air reservoir will be made of 3/4 in PVC pipes and fittings. I haven't determined the exact size, but I know it won't be that big. It's relatively low pressure for now, so I don't expect much. I'd be happy with 1-20 shots per fill.

I'll probably increase the pressure once I make the design work.

A good track pump is not that expensive and will give you 250 psi or so with relative easy, much more power/shots and still within the range of what PVC can safely take.

OK. I'll try to find one at the local store. Thanks!

All I have right now is a cheap air compressor for bicycle tires that goes a little over 100PSI and a CO2 tire inflator already in use with another project, based on this one you made:

http://www.spudfiles.com/forums/6mm-sem ... 10075.html

It works very well.

Binary Blue wrote:It works very well.

This forum has a showcase section

I just have to build a case for the semi-auto before I can show it off. It's just very bare bones right now.

I'm trying to make it fit into this shape:


Just some cutting left to do on my CNC mill.

Also, your post said something about high friction in that design. I found that lubricating the inside of a syringe with vaseline (I'll try silicone lube next) cuts down a lot of the friction. In fact, I was able to use the syringe's plunger instead of casting one out of epoxy.

Petroleum jelly should be safe for use in the syringe since syringes are usually made of the same plastic that petroleum jelly containers are made of and are usually designed to be chemical-safe.

Binary Blue wrote:Just some cutting left to do on my CNC mill.

I underestimated your facilities - if you have that available, why not make your design fit into a block into which all the parts (barrel, chamber etc.) plug into?

Petroleum jelly should be safe for use in the syringe since syringes are usually made of the same plastic that petroleum jelly containers are made of and are usually designed to be chemical-safe.

The issue is the plunger seal, it tends to absorb lubricant and expand, creating more friction. Ah well, now that I have my own lathe and mill, my epoxy piston casting days are long behind me

I prefer building things using ready-made parts for now since they will likely be better than what I can machine at home right now.

Also I don't have access to anything to cut besides lots of wood and very thin particleboard right now.

But I plan on using the CNC machine to cut the hole in the PVC endcap.