that is the very reason I steered away from the .243/6mm in the first place. For some unbeknown reason they can be very fussy on projectiles. A friend brought his girlfriend a beautiful Ruger No1B in .243. With the Speer 85 grain bullet it would rip out one tiny little bug sized hole for five shots. A 100 grain bullet it was lucky to put them onto a dinner plate at 100 yards
. It's not only twist rate as related in the Greenhill formula but also the altitude also makes a difference as a friend has just reported on another site.
he has been playing with 60 grain .22 L/R ammunition from Aquila and he lives almost at sea level. His friends who live at around 8000 ft in Colorado find it's shoots well in most of their rifles but to get his to stabilise he needs a special custom dast twist barrel. Temp can also effect it. Internal ballistics seem more akin to Voodo than science at times.
Oh i know have both a .243 adn a 6mm Rem chambered rifle
.