I just heard back from Palmer, and the official OD of the black powder coated barrel is 0.812 or 13/16.

Rogue42, you were pretty damn close! (the second time anyway )

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Tippmann A5 w/ Response Trigger - Apex Tip + UMS Apex Adapter + 12" Black Palmer Brass - 20oz CO2 on Remote w/ ProLine Pro Connect - Palmer A5 Stabilizer Kit - 8" Skeleton Stock - TechT Vortex and Lightening Bolt Mod - X7 Hopper - Madman Spring Kit - Lapco Offset Sight Mount - UMS Picatinny Grip - UMS A5 RIS MkII Front Tactical Mount