Could I make a silencer for my air canon?

Having a longer barrel would reduce noise. Whether or not that will effect the trajectory and speed would have to be tested. If you plan on building something as you described be sure that you are wearing eye protection when testing, and that people and animals are a safe distance away.

In regards to creating a removable silencer, be sure you understand that just possessing one, even a home made one for use on an air cannon- disposable or not- can get you into a bit of trouble.

This short read explains how seriously ATF takes any type of silencer. Sure you are not likely to be caught, however a stroke of bad luck can make you wish you had not attempted it at all.

@iBite True, I’ll have to see what the laws are in canada for that.

Reallu I’m more wondering if the concept would work the same for this as it does in a real gun, just scaled up a bit. I’ll probably try anyways, but it would cost a decent amount for the PVC since I need to buy the whole length, and cut it in half accurately. Also drilling ¾” holes in metal since I need to buy a bit tht big.

It seems to me that you are not getting a very good seal between the projectile and the barrel.
The escaping air in the gap might be the cause for the dreadful noise. (the gap also decreases projectile velocity by a tremendous amount).
Try closing the gap by using a piece of cloth, like they used for those old muskets and spherical lead bullets.
I also do not think you should use batteries as projectiles, as they are full of quite strong acid.

@ragingloli Yeah, ?I use a chunk of spray foam, I also have a different projectile now, a peice of dowel with copper ppe cap

Nicely done – and I’m glad you recognized the 70 psi limitation of the solenoid so you don’t hurt yourself. We use 220 psi pipes for our potato cannons.
At first blush it looks like your compressed air storage volume is many times greater than the volume in the barrel. That large quantity of elevated pressure air will make noise when it escapes. You can play with the sizing. That will help.
Is the solenoid bidirectional? You could rig up a 555 timer chip and an adjustment pot so that you drive the solenoid closed just before the projectile leaves the barrel. The advantages would be: quiet, no waste, and faster reloads.
If the solenoid is spring loaded in one direction you can rig up the 555 to connect for a limited time. How long? Using Physics 101, figure out the muzzle velocity. Then use v^2=2as to calculate the acceleration rate a. Then use V=at to get the time. It will be perfect.
How do you get muzzle velocity? Shoot it up into the air and time the total flight from “fwoomp” to “plop”. Divide by 2 and multiply the time by g, gravity.

I realize you might not know what I was talking about so I will do an example for you. This will be on the Physics test, so take notes.
Problem: Assume the barrel is 2 m long and the total flight of the projectile is 8 seconds when shot directly overhead.
a) Find the peak height of the object.
b) Find initial velocity
c) Find the acceleration
d) How long should the solenoid be open for max power and least noise?

a) Height of the object: Using s= ½ at^2. t is half of 8, or 4sec, a is g = 9.8 m/sec2 Call it 10 m/sec2 Height is s= 0.5 * 10 * 4^2= 80 meters
b) Initial velocity: Using V=a * t V= 10 * 4 = 40 m/s.
c) Determine acceleration: V^2=2as, 40^2 = 2 a 2m, 1600 = 4 a, a = 400 m/sec2, a= 40 g
d) Solenoid open time. s= ½ a t^2 2 m = 0.5 * 10 t^2 2 = 5 t^2 t^2= 0.4 t= 0.63 sec.

Extra credit: If you know the mass of the projectile and the cross section area of the tube you can determine max pressure.
If you know the firing angle you can determine maximum range. Answer: At 45 degrees – 160m.
Isn’t Physics great? If you learn this stuff now you’ll use it for the rest of your life. Honest.