I tried just about every suggestion here, but still couldn’t get an airtight print. This is transparent PLA. I’m going to play with my speeds and overlap to try to get it clearer, but it was dimensionally accurate and completely sealed the first try.

Don’t look too carefully at the barrel, it’s just a hollow test one. I was able to eyedropper fill and test the grip section with it, but the OD is wrong, and there’s no real filling mechanism in it. That’s phase 2 of this project!