wow, 14!?
I'm pretty sure it's not the driver, I may try to figure it out again when I'm not so busy.
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My first spud gun (combustion) specs:
Name: "A"
Fuel: Propane
Ignition: Flint, Lantern Lighter
Barrel: 2" x 40" - 125.6 Cu
Chamber: 4" x 12" - 150.7
C:B Ratio: 1.5:1
Weight: 5.8 Lbs.
Here's a few pics that I took today
Spud gun against my wall
The Chamber
Lantern Lighter Igntion