Propane gun build

[Mb01]

Before I build I want to see if my calculations are correct. If I start with the 1.5:1 ratio, I will have a 4 inch x 18 inch chamber and a 2 inch by 4 foot barrel.
Volume of the chamber would be 226 cu in., which is 3,703.5 cc.
Just to make numbers easy, a 5% propane syringe injection would be roughly 185 cc. That seems like a lot. Basically 3 full 60cc syringes.
Am I missing something??
I haven’t built one since the 1990s, and we used Aqua net back then but it gummed up everything. Looking for a clean burn with some pop. I was sold on 99% isopropyl alcohol for this build, but I have just started thinking about using propane instead. Thoughts?