I am putting together a .358 STA on a Rem. action and will be using a PacNor bbl.. I would like a 12 twist -- and PacNor makes it--but only in a 5 groove bbl.. My question is -- would there be any advantage in a 6 groove bbl. over a 5 groove or any other configuration for that matter?

No.

None

Zero-Zip-Nada

With a 6 groove, you can slug the bore and measure the slug for groove diameter easily, but with a 5 groove you can't(unless the rifling land is really narrow). Since you are not talking about a military gun bore with doubtful groove diameter(like SMLE), this is totally irrelevant.

Given the same height and width in rifling lands ,I would say that 5 groove disturb bullet integrity less because there's less metal sticking out in the way of the bullet. In 4 groove or 6 groove bores, lands opposite each other, thus some part of the bullet are compressed by pairs of two opposing lands. On the other hand, in a 5 groove bore, land and groove oppose one another thus all the compression in the bullet by a land is some what relieved in the groove area across.

Other than those, I can only think of NIL