Geez guys, where's your creativity?
You don't need to buy fittings! Why would you for a gun that won't even be generating over 90psi or so? Cast your own caps and reducers out of fiberglass.
Get the longest chunk of the biggest pipe you can find to use as the chamber, and then reduce down to 8" so that your ammo doesn't weigh hundreds of pounds.
Inline will make for more power, and it doesn't seem as though high portability is a priority here. It also means much less complex fiberglass castings.
As for ammo, cast ice or concrete always works, or shotgun type loads of vegetables with towels for wadding.
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