thoughts on valve cylinder materials and porting

[MrDEB]

Am in process of designing an air cannon using a QDV set up. Aquired a large propane tank for free (4 ft tall and about 16 inches diameter).
plan is to install a 2 inch galvanized pipe down the middle for the valve cylinder but was wondering if 2 inch exhaust pipe would work as well?. for sure cheaper and maybe smoother ID.
on subject of the three or four ports, if the ports are square verses round? thinking inline with a high performance engine with ported heads.
Now when the piston is moved back to clear the ports, will the void cause turbulence within the ports? the piston moves back further down the cylinder thus leaving a void.
just thinking out loud.
If using exhaust pipe, then brazing onto the propane tank may be an option??