Help with large combustion cannon

[Cthulhu]

I'm making a combustion howitzer that has a 3 inch PVC barrel that is 6 feet long as the barrel, I know that I need a 1.5:1 ratio combustion chamber, but my main concern is heat. Will my Spud gun melt on the inside from this enormous combustion?
I also need to know if there are any tips for building these large cannons, such as sparker placement or reinforcing certain parts.
Thank you so much for your help, I will post pictures when it is completed.

[jimmy101]

Your gun won't even warm up noticeably let alone melt. The combustion process generates a high temperature but relatively little heat.