Cartridge hybrid build log

[Skywalker]

Well, JSR et al have been bemoaning the lack of innovation on the spud forums; I've been itching to build something, and now that I'm on winter break, I've got a project.

My target is to use 5/16" brakeline for the barrel, which is something between 0.25" and 0.22" ID, so I can use .22" airgun pellets and just flare the tails a bit a la JSR. The chamber is going to be 3/4"M-type copper, probably 5" long.

The catch is how to make the union. My favorite idea is to use the existing flare couplings from the brakeline, but unfortunately they only sell the fittings attached to the brakeline. So, I'm trying to wrangle some 3/8" compression fittings from the local plumbing supply into serving the same purpose. The basic idea is to turn the fitting backwards, epoxy the compression nut into the chamber, drill out the male part to accept the barrel, and glue a short stub of the brakeline into it.

I've got a couple ideas about how to make the schrader/ignition work on the rear of the cartridge. Fueling via syringe.

Anyway, I'll try to put up some images and/or drawings as I go. I've also got some iron fittings for a single-shot hybrid in case this doesn't work out in the time I have. As far as a gun for the cartridge system, I'd like to make it semi-auto, but I've never done anything like this so I'm not going to baseline that aspect -- bolt-action is an acceptable outcome.

[jackssmirkingrevenge]

Semi-auto is more than feasible, if I managed full auto with a pneumatic cartridge project then a hybrid has a lot more power potential to cycle the action.

[Skywalker]

I suppose I should have added a caveat to that statement, concerning my abilities and the time I have available over break. I'm sure it's doable -- in fact, I'm pretty much just copying your homework on this one.

The first prototype is curing a.t.m. I need to get thinner epoxy, and quicker-setting too. I also need to make a jig to center the parts as they cure, the barrel stub is off-center and angled.

Here's the cartridge with the 'union' unscrewed. The 3/8" compression nut on the right gets glued into the cartridge, and then the male part threads into it. The barrel stub is epoxied into the drilled out male adaptor.



Here's the assembled cartridge.



I don't have a good image of the schraeder/sparkplug end. Suffice it to say, the schraeder is glued into a plastic insulating sleeve, which is glued into the copper endcap. A segment of paperclip wire is squeezed between the plastic sleeve and the copper, and forms the spark-gap with the tip of the schraeder.



EDIT: I resorted to putting them on flickr.

[jackssmirkingrevenge]

Shame we can't see the pics, care to post the url?

[Skywalker]

JSR: okey-doke, I fixed it. Thanks for inquiring!
Ilovefire deserves a lot of credit for this too, obviously-- I'd seen his design and basically reworked it to use the materials I have.

I'm working making the rest of the cartridges with the schrader recessed into the rear of the chamber, using a washer and lots of epoxy instead of the endcap. I also need to make a jig to center the compression nut while the epoxy cures -- this one is not quite aligned. I'm also working mocking up a simple gun to fire the cartridge.

I test-fired it twice; once blank, once with a 1/4" steel bb in a 24" barrel. It penetrated a juice bottle full of water, making the bottle jump several inches in the air -- I need to do more testing, but the fuel-air mixture ratio is hit-or-miss right now, b/c I don't have a shock pump yet.

[jackssmirkingrevenge]

A bit bulky but looking good, what mixes are you firing at? Looks like they'd work well for a bolt action, that endcap rim would give an extractor something to grip.

[Skywalker]

They are bulky, but I wanted plenty of oomph (or in layman's terms, MOAR POWR!) As to mixes, that's the trouble -- I'm not really sure, b/c my pump doesn't have a pressure gauge. It's sort of a miracle that I managed to get it to fire at all.

Yeah, I thought about the rim as a contact point for an extractor, but I decided the schraeder is likely to get broken off, and it may also catch on the way out of the gun -- it sticks out almost 1/2".

[ilovefire]

Ilovefire deserves a lot of credit for this too, obviously-- I'd seen his design and basically reworked it to use the materials I have.

thank you, but im going to have to say that it way pretty much all from JSR's thread on his hybrid cartridge project, so you really need to thank him. some people think its strange but i dont really like taking credit for what i didnt do.

[jackssmirkingrevenge]

my pump doesn't have a pressure gauge. It's sort of a miracle that I managed to get it to fire at all.

Actually, it is!

Yeah, I thought about the rim as a contact point for an extractor, but I decided the schrader is likely to get broken off, and it may also catch on the way out of the gun -- it sticks out almost 1/2"

Dropping a schrader on a hard surface, especially with a heavy cartridge attached to it, will almost certainly result in deformation that will make it impossible to seal against the pump, so that's a smart move.

it way pretty much all from JSR's thread on his hybrid cartridge project

... which was inspired by CpTn_lAw's mini, the line of credit can go on forever

[Lockednloaded]

i've made a similar union, but had trouble getting it to seal. What are you using as burst disks?

[Skywalker]

I had best luck with packing tape (for strength) and electrician's tape (which is soft, for a good seal) stuck sticky-sides-together. Flat sealing faces or rubber washers help.

I started construction of my own high pressure pump, but unfortunately, I'm now back at school, so this project is officially shelved indefinitely.

[metalmeltr]

Just so you know they do sell flare nuts at the auto parts store, just ask typically in the back near the line

[jackssmirkingrevenge]

this project is officially shelved indefinitely.

Shame, it was looking promising.

[Skywalker]

@metalmeltr: yes, but only the nuts, and not the female part. I'd have to buy another length of brakeline to get two more of those, so it comes out cheaper to use the plumbing fittings.
@JSR: thanks man! I agree -- I just don't know when I'll be home again -- graduate school does that. I don't think I'm going to find a rental apartment where I can have a shop and shoot noisy things.

[Gun Freak]

@metalmeltr: yes, but only the nuts, and not the female part.