SpudFiles

The ported length of barrel can be counted as part of the barrel length. If noise is a concern, you could definitely go for something lower, say 0.5:1 - you won't lose out on much power, and it will be much quieter.

Usually, perforating the whole length of the barrel is done when you have a cartridge that would normally be supersonic but you want to get it down to subsonic speeds. I would port the final 6 to 8 inches of barrel, sleeve that with as big a diameter pipe as you can, and pack it with a suitable mesh coated with lithium grease.

Oh ok I see, should I worry about the grease ever drying up and making chunks over time and blocking the holes? Also, how big should I drill the holes, barrel is 1/2'' PVC. Another thing I'm worried about is that I plan on using a ram rod to load the cannon, usually with pneumatics I would throw in the marble followed by a piece of tissue so the marble doesn't fall back out, should I worry about pieces of tissues getting stuck in the suppressor holes?

The grease should last a long time. Even if it blocks the holes, the blast will blow it out. Same goes for the scraps of tissue, the gasses will burn them off

For a 1/2" barrel, I would go for 1/8" holes, but lots of them. Remember it takes 16 1/8" holes to have the same area as a single 1/2" hole!

any downsides to bigger holes? I mean 1/8'' is just... tiny

You will weaken the barrel more, though I guess for a combustion it's not a big deal. Don't go above 1/4" though.

I'm pretty sure I've asked this somewhere before but, does the I.D of the chamber actually matter so long as the volume is in correct C:B ratio? I'm thinking about what you said and I might go for a .75:1 C:B ratio, does it matter if that .75 is in 1'' PVC or 1 1/4'' PVC?

Yes it matters, a short fat chamber will out perform a long thin one. Don't know if 1" compared to 1 1/4" is noticeable though.

I only need 3.5 cubic inches for the chamber so would the difference between a 1'' chamber or a 2'' chamber be significant for such a small chamber volume?

3.5 inch3?? That's 0.056l... Correct me if I'm wrong. If not, that's way too small to be anything interesting (as a combustion) you probably won't notice a difference, the shot is going to sound like a fart anyway

Well, it's a 1/2'' x 24'' barrel, assuming I.D actually is 1/2'', the volume would be ~4.7 cubic inches, if I wanted a .75:1 C:B ratio it'd be (4.7*.75) = 3.5? Basically I want something like this what do you think the dimensions for that would be?

Your calculation is correct, approx 3.5 cubic inches.

hmm is wyz right? Is this project even worth pursuing? Will any power come from such a small cannon? I'm only looking to shoot ~40 yards, if my idea can't get that, what do you guys think would help me get that distance with good velocity in a small form factor (while keeping barrel at 1/2'')?

Define good velocity, and with what ammo? I've had things that use more fuel than your chamber volume so our ideas may be different. But I'm more familiar with pneumatic. With a paper dart it may fly 40 yard with it, with a marble is more likely to roll out of barrel though, you need at least 1L chamber. All just my thoughts though, you should ask somebody who is familiar with combustion to help you.

I was hoping 300-400 fps range with 1/2" marbles, but I'm not all too familiar with combustions either, thanks for the input though ☺

I'd just build it and see what happens. At worst, you'll have made yourself a little plinker/measuring stick for future projects. With a combustion, you can just reuse the igniter when you want to scale up, so you'll have spent like $10 on PVC.