I did end up using CAD to get my measurement, but not quite this way. Had to go back to some secondary school geometry and figured out how to unwrap the helical path. If anyone else wants to do the same process, you need the radius of the cylinder (R), the height of the cylinder (H), and the length of one of the helix edges (L). After a bit of math, you get: p = 2HπR / sqrt(L² − H²)

For the Pen15mm barrel STEP, this lands less than a thou from a 7 inch pitch (or 177.8mm). Strangely enough, the projectiles from the 15mm Handbook don't match up exactly and give a pitch of 176.04mm. Not sure if there are any implications to this.

TDLR: The apparent standard for 15mm barrels is a 1:7 inch twist.