Paintball/Conical Springer help

[octocamo]

Hello i'm new to this forum, been reading it a lot of times, but never registered. Now that i have some spare time i was on the bench making drawings of a "diy marker", mainly because markers in my country cost from 300 dollars OEM each one. And not talking about a Tippman or anything like that. The project is basically gather with my friends and build a simple yet effective weapon capable of firing .68 paintballs in a semi automatic fashion. More or less like Nerf PlusBow, but for paintballs and a feeder.
I made some schematics, but my problem is basically the spring part, how can i know what spring or springs are needed for the task?
What is better? compressing a piston and shooting the pellet with the compressed air or just pushing the pellet out of the barrel in a slingshot/crossbow fashion?
Please light me on this quest.
edit: i forgot to talk about the range or fps i want, something on the likes of 150 fps is what i want, in the range of slingshot paintball. Also the straight range desired is like 10-15 meters (30-45 feet)
And i'm willing to document the building process and post pics. =)

[roboman]

More or less like Nerf PlusBow, but for paintballs and a feeder.

+Bows aren't at all semi-automatic...I think you're referring to the ABP5K, made by the same person.

If you have an air compressor, or even a bike pump, I would recommend a simple valve hooked up to a barrel and an air tank...may be a ball valve? I did this once, out of boredom, and I found that 3/4" CPVC makes a remarkable barrel for .68 cal. paintballs.

[jackssmirkingrevenge]

Have you seen the Airrow launcher?

It basically turns a bow into the powerplant for a piston airgun, it would be a good place to start your research, especially if you have bows available. I had made a smaller version for airsoft BBs as a school project a decade ago, it would be easy to scale up for paintball and add a simple tee magazine so it can be reloaded automatically.

[roboman]

That is simply incredible. I may have to acquire one...now.

[jackssmirkingrevenge]

If you have an air compressor, or even a bike pump, I would recommend a simple valve hooked up to a barrel and an air tank...may be a ball valve? I did this once, out of boredom, and I found that 3/4" CPVC makes a remarkable barrel for .68 cal. paintballs.

For the sort of power you need for paintball, a blowgun hooked up to a reasonably large airtank at 100 or so psi is probably sufficient for a game. Still a spring/bow powered launcher has the advantage of not needing any ancillary equipment.

[octocamo]

@roboman:
the idea is to have something to use on the field, like a Airrow as jackssmirkingrevenge says (that was difficult to type).
and the 3/4 inch pvc is a perfect barrel for this things because it gives space for lube and paintballs aren't at all .68 100% of the time.

@jackssmirkingrevenge:
no bows available, and the airow is a cool thing. but made me wonder, what is more effective? compressing gas or shooting the pellet with direct energy of the spring?
Aside from that, that thing you made is the kind of thing i tend to make, simple an dangerous hahahah

[jackssmirkingrevenge]

as jackssmirkingrevenge says (that was difficult to type).

JSR usually suffices

no bows available, and the airow is a cool thing. but made me wonder, what is more effective? compressing gas or shooting the pellet with direct energy of the spring?

If shooting it directly was a good idea, we would see less spring piston airguns and more stone bows.

You have various options for powering the piston, bows, coil springs, gas rams, it depends what you have available. A bow is easy to make using piano wire as I did in my airsoft crossbow, but of course is less compact than a coil spring.

Have a look at these projects, you might get some ideas

Also, this demo is a good illustration of how a commercial spring piston rifle works.

[octocamo]

thanks for the links jsr, i am now convinced with the pistons, orings and stuff. this is what i bought today to make some testing:
1 meter (3 feet and 4 inches) of 3/4 inch hot water tubing.
half meter of 1/2 cm rubber tubing (used in slingshots)
half meter of 3/4 cm rubber tubing
and i've got a thin pvc tube that fits perfectly inside de 3/4 hot water tubing.

ideas?
PS: i know this is ghetto, but you know, lexan here is 10 times expensive as it's on the US or europe. but if it turns out cool and useful, there's no doubt it will become lexanific and deluxant
edit: i realised that is really hard working with rubber bands, they are so unpredictable...
on the other hand, i was thinking on making a chamber with a bigger tube, like saying a 2 or 3 inch one, and a fitting to connect it with the 3/4 inch one. Then a piston to make it work. Any thoughts on the correct piston size or spring power needed? I'm quite good at math ( i study ingenieering) so any EQs will be ok. A link to a something related to this kind of project would be awesome.
Again, this forum is great, i must say so hahaha.