Cleanout plugs are almost always only DWV rated. PWS systems don't need cleanouts so why make'm or stock'm? To put it another way, walk into a new construction and examine the pipeing. The DWV system probably has at least one cleanout plug in it somewhere, particularly in the "drain / waste" part of the system. The pressurized water (potable water, PWS) system won't have any cleanouts. The local pipe supplier carries the DWV cleanouts but probably not the PWS ones.
A DWV only cleanout is fine for combustion guns at 1X mix. Just make sure the threads of both parts are decent and that the cap is screwed on at least a couple full turns.
I would say that 99%<sup>*</sup> of all combustion spudguns are built with DWV cleanouts.
<sup>*</sup> 84.263% of all statistics are made up.
Need help
Well, before you go further, consider changing your barrel to 2" instead of 1 1/2". It will be shorter. Another possibility is to reduce to 2 1/2" then use tennis balls or instead of glue connections use threaded and have both barrels. Remember, the sweet spot for tennis balls is .7 to 1.