Potato cannon pipe sizes? (spud gun)?

for best velocity you'd want a combustion chamber at least 3X the volume of the barrel.

the chamber shouldn't be more than 1/3 the length of the barrel.

so for a 1" barrel a 3" or 4" chamber would be ideal.

you'd also need to use a good quality gas or this won't work, a good gas such as propane or butane. roughly 35% gas, 65% air. if you could, inject the fuel in liquid form, to prompt it to mix itself with the oxygen inside.

-------------

my best spud gun has an accurate range of about a 2 miles. it shoots a D cell battery from a breech loading hybrid spud gun using either butane or propane gas mixed with oxygen gas at high pressures.

the bore is about 1.2", Damascus barrel-style bore construction with a barrel made of multiple layers of tubes and fibers. the chamber is 3" diameter and 15" long. the barrel is 6 feet long.

It uses a Yugo SKS grenade sight for aiming and further correction is by spotter with high power telescope. point-blank range is close to 400 yards.

my highest velocity and most accurate spud gun is my AA battery hybrid, it shoots a AA battery from a breech loading bolt action repeating (uses a box magazine of 5 rounds), automatic electro-mechanical metering of pure oxygen and pure acetylene gas between shots. there's a button to fire and a button to recharge the chamber. between shots the bolt needs to be cycled and a new primer put in the priming hole. without these, the gas will come out of the chamber as the gun is being charged.

Bore diameter is 0.582, with 3 lands & grooves cut into the bore. barrel is 5 feet long with a 1 foot x 3.5" combustion chamber.

this gun uses a modified Yugo SKS grenade sight with magnification for long range, it has an effective range of about 600 yards and a point-blank range of about 300 yards. the velocity is estimated at about 1500 fps.

Propane Potato Gun

Use a rain gutter with a vegetable can duct taped to the bottom

There is a huge amount of information on spud guns available. Google it.