How would I replace my 2" ID PVC with a 2" ID aluminum tube?
If you don't have machining facilities,
epoxy casting would be my recommended route - in this case epoxying the pipe into a threaded fitting, perhaps a
union to make it replaceable.
I would attach pictures but don't see an easy way to do that.
1) Click on the "Upload attachment" link below the post text box
2) Click on "Browse..." and select the image you want to upload
3) Click on "Add the file"
I really like the idea of adding the 4" pvc as a sleeve over the 2" barrel. How large should the ports be? How many ports.
As many and as large as possible, but not exceeding the length of the sabot. For this concept to function, it is essential that the air in the barrel remains as contained as possible in the sleeve:
Is the sleeve sealed at both ends?
For the same reasons articulated above, you need the air not to escape the sleeve.
What would happen if I only drilled ports in the 2" pvc and didn't use the sleeve?
The air would exit the barrel and stop accelerating the sabot, but since you haven't created a "suction" effect, it would not be trying to brake the sabot.
The best way to illustrate the idea is to press your finger against the nozzle of a syringe and pull the plunger. You'll note that there is a force trying to push the plunger back in, because atmospheric pressure is greater than the pressure inside the syringe chamber - this is the effect we are after. Otherwise if you only rely on barrel friction, you're going to end up with a ridiculously long barrel that is completely impractical.
If I add the sleeve I believe I can do the same thing, gluing the 2" x 4" adapter to the lower end, gluing the first section of 4" sleeve to the adapter, connecting the 2nd section of 4" pvc with a hub (no glue) and gluing the top end 2"x4" adapter to both the 2" PVC and the 4" PVC sleeve. The weak point will be the midline barrel/sleeve hubs that are not glued.
Since the integrity of the sleeve is paramount, I don't think you can really separate it.
That being said, all you want is the spare volume. You could for example epoxy a couple of bottle caps into the sleeve and when you arrive at your destination, screw in 3L bottle et voila, tons of spare volume that you don't have to pack in your bags:
edit:
Screw having a sleeve at all, why not fabricate a central hub that you screw several 3L bottles into?
Looks a bit silly but pretty is as pretty does, you have your expansion chamber cheap at destination without eating into your baggage.
Another point worth making, this system will work much better with a combustion as opposed to a pneumatic launcher. Unlike pneumatics, in combustions the heat generated is an important contributor to the pressure, and as the gasses expand into your sleeve, they lose temperature and therefore pressure more rapidly. Of course it depends where you are, I would imagine combustions are a no-no in most areas.