The term polygonal is rather loosely used by barrel makers these days and I'm not going to get into fine definitions.
"Wasn't there a 'poly-something or other' artillery gun made during the civil war?
iirc, they called the projectile a 'bolt', and it was twisted to the same rifling as the barrel."
Yes, that was the Whitworth true hexagonal bore field piece. There were also .451 rifles on the plan, famed in song and story as "Confederate Snipers." What is less known is that the Whitworth hex bore (and the pentagonal and heptagonal bores cut by companies intent on cashing in on the gimmick without running into patent lawsuits) was obsolete within about five years. The land and groove Gibbs, Metford, and Rigby rifing took over for the remainder of the muzzleloading bullet firing rifle era. I believe it was the Rigby that Remington and Sharps breechloaders beat at Creedmoor in 1876.