Yet Another Noob...

[origin unknown]

I would go with a black iron QEV (Quick exhaust valve) cannon that you just screw the fittings together with teflon tape which is really easy to do. The only hard part is finding the QEV, but luckily I have done the work for you. They work the same as a sprinkler valve, but are designed for compressed air, not water. You can get one from here: http://www.fastenal.com/web/products.ex?N=999603556, just sign up with this company and you can order a QEV for real cheap. The one that says 1/2" tube x tube has 1/2" threads likewise the one that says 3/4" tube x tube has 3/4" threads. I would go with the 1/2" or the 3/4" QEV, the 3/4" is better if you wanna be shooting larger projectiles. When you use fittings of any kind that have threads, USE TEFLON TAPE. You can get teflon tape in your local hardware store. Here is an example of a black iron QEV cannon to show you something similar to what I am talking about: http://www.spudfiles.com/forums/the-bla ... -t241.html
You can make yours look more like a gun by having a single chamber. You can also make a barrel from pvc to use with a QEV, it doesn't have to be just metal. If you want power, not noise, I would go with a pneumatic. You can also just make a simple sprinkler valve cannon for your first.

[SnowFlox]

Combustion is much more easier to build then a pneumatic i think.

I just only build combustions by the way.

Only negative thing about combustion is the fuel ratio.
with Pneumatic you dont have that problem.

[Pilgrimman]

Pneumatics are not difficult to make, even in comparison to combustions, IMO. This is assuming that you want to be basic in your design. All you need to do to make a pneumatic work is to use pressure rated materials, and make your gun airtight. This is much easier than it sounds. I'd check out the solvent welding course on this forum if you've never used PVC, and if you solvent weld decently, you'll never have leaks. I personally have never had a leak in my life. The only thing I would consider even slightly difficult is mounting the fill valve (Usually a Schrader valve). You can pick one up at an auto parts store for about 3 USD. For simplicity, it would probably be best that you use an un-modified solenoid sprinkler valve. These are quite easy to modify later on, when you feel confident enough. I admit, I am biased toward pneumatics, but, in terms of power, they blow combustion guns out of the water. I also think that they are cheaper, as air is absolutely free! I'm all for noise and effects, but combustion guns just don't do it for me. If you want a gun that will chuck just about anything well out of sight, I'd highly recommend pneumatics. PM me if you've got questions.

[jimmy101]

I also think that they are cheaper, as air is absolutely free!

Air is free but shoving it into a small volume isn't. Even a cheapo shop compressor is what, $100?

EDIT: Yes, I know, the original poster said he already has a compressor. Still, compressed air guns are not very portable.

[Peter Cottontail]

Im in the same boat, i have my good old combustion(1.5:1 CB) hurls a potato up to 1/4 mile and sounds like a 12 gauge, and now im feeling like making another. Im debating between a smaller more portable combustion or a simple pneumatic(potentially a ball valve one since i dont have a lot of money) and yea.

[DYI]

440 yards is quite a range claim. I'd like to see a picture of this cannon.
It takes a lot of force to fire a potato that far. I would agree that this cannon wouldn't be very portable, since, for a simple combustion to fire a potato that far, the barrel would probably need to be more than 10' long.

As for cheap pneumatic valves, burst disks are about as cheap (and powerful) as it gets, and piston valves can be built for a reasonable price if you have access to pressure rated plastic pipe. Ball valves make firing more difficult and can't be opened anywhere near quickly enough (even with pneumatic actuation) to compete with piston valves or burst disks.

[origin unknown]

I also think that they are cheaper, as air is absolutely free!

Air is free but shoving it into a small volume isn't. Even a cheapo shop compressor is what, $100?

EDIT: Yes, I know, the original poster said he already has a compressor. Still, compressed air guns are not very portable.

I made a compressed air gun that is only 6 1/2" tall. Call that unportable! It is a burst disk and you just pump it with a small bike pump, and it can shoot a battery about 150 feet strait up in the air!

[Peter Cottontail]

To DYI: right now its in my buddys garage, but my best descripsion in words is HUGE, for example, our barrel is somewhere around 7 feet long(2in diam.) and the chamber is about 5ish feet long(4in diam). Now add propane and such and pre cut potatos(we took some 2in pipe and made a mold so we could pre cut potatos and cut the tips to make them fly a little better.)
The problem is that no1 can simply just load it, pick it up, and fire since it needs support so it dosnt bend. Building huge cannons is fun cept it goes thru propane fast and my neighbors arent to hot about it.

[DYI]

Well, with metered propane and the dimensions you quote, it seems at least plausible, if not slightly overestimated.
I really hate wobbly barrels, they triple the setup time before firing, and you can never seem to get them completely straight. I have a 12' ABS barrel for my SCTBDC , and while it provides loads of power, I usually just use the 4' barrel due to decreased setup time.
EDIT: just read your second post more carefully, and discovered it directly contradicts your first. In your first you said that the launcher had a C:B of 1.5:1, which would mean a 2 inch barrel slightly over 13' long, and, in your second, you said that it had a 2 inch barrel that was 7' long.
Which one is true? With such a small barrel, the performance would likely be very low.

[SpudBlaster15]

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[jimmy101]

pre cut potatos(we took some 2in pipe and made a mold so we could pre cut potatos and cut the tips to make them fly a little better.)
The problem is that no1 can simply just load it, pick it up, and fire since it needs support so it dosnt bend. Building huge cannons is fun cept it goes thru propane fast and my neighbors arent to hot about it.

It doesn't matter how you cut the spud it'll fly the same way, poorly. Any simple geometric shape of constant density material is aerodynamically unstable. Cylinder, sphere, brick, bullet, ... doesn't matter, all are unstable in flight.

A sharp nose won't help a subsonic round, even a fin stabilized or a spinning round won't be helped by a sharp nose. Take a look at a commercial jet, the nose is shaped about like a spud. Only supersonic aircraft need a pointy nose.

;:;:;:

I wonder if you could stiffen a long barrel by taking a matching piece of pipe, slicing it in half lengthwise then using copious amounts of a suitable glue to glue the pipe halves onto the barrel. The ID of the outer pipe won't match the OD of the barrel so the glue would just be along the edge of the outer pipe halves.