To be honest, I don't have a good idea of what size is considered a tight bore either.
According to this article:
6mm Bore Diameter and Bullet Selection « AccurateShooter.com Bulletin
It gives the impression that bore sizes range from .236" to .237", so .236" would be minimal.
Kriegers site lists .236" or .237" options
Calibers & Prices
According to the above, it would seem that PacNors .236" may not be radically tight, but certainly on the tight side. The difference between .236" and .237" may not seem like much, but I'm willing to bet it makes a big difference regarding the heat generated from friction as the bullet scrapes down the tube.
It might be a good idea to specify a .237" bore if you build another 6mm-284. Maybe you can ask someone who's not having bullet failures if they know their bore size.