Don't use sabot bullets. ( ok there aren't many about in .243 / 6mm)
An absolute must is to ensure that your bullets are stable, the correct weight and velocity for your rifles twist rate. Test groups before fitting and moderator. if you are getting keyholing on the target, to not use that ammo.
Also if you are pushing a frangible bullet very hard and very fast, and it maybe reaching the point of disintegration, chances are if you fire it through a mod, the increase in back pressure will invarable be the straw that broke the donkeys back and may come apart in them mod. A bit of common sense on bullet choice, twist rates and velocity keeps you and your mod on the safe side.
lets say you had a .223 Rem with a 1 in 12" twist, I wouldn't recomend shooting 70grain + bullets through a mod. chances are they are unstable directly in front of the muzzle and will clip a baffle.
or a .308win with a 1 in 12" twist. I wouldn't recomend shooting 200 - 220gr bullets, especialy at subsonic velocity chances are they are unstable and will clip a baffle.
or a 6.5x55 with 1 in 9.5" twist, I wouldn't recomend shooting 160gr bullets. same story.
There is a relationship between the clearance of the baffle bore, the turbulance caused by propellant gasses excellerating past the bullet, certain types of bullet bieng effected by the turbulance more than others, and innaccuracy( or even catastrophic failure in the worst case senario) when using a moderator, or indeed a muzzle brake,
Basicaly what it amounts to is that a moderator (or a muzzle brake) needs enough clearance in the bore of the baffles, and the baffles need to be of the right geometry, and the correct distance apart to counter effect any negative influence on the projectile(bullet) during the time of its flight through the muzzle attachment-moderator or brake.
very tight clearances on the baffle bore to bullet diameter are a recipie for disaster for several reasons.
you never know if a factory rifle barrel bore is 100% in the middle of the barrel
you never know if the muzzle therad is 100% inline with the axis of the bore
from experience it pays to have at least 1.5mm clearance in the bore of the baffles above the size of the bullet to be used. ( so a 6.5mm bullet would idealy have a baffle bore diameter of 8mm)
firing a smaller bullet, say a 5.56mm ( .223) through the same 8mm bore diameter of baffle gives you 2.5mm clearance ( or near enough) and providing the geometry of the baffles is correct, and the baffles are the
correct distance apart,
there is no noticable loss on sound reduction.
There is a mountain of theory and practice behind sound suppressor / moderator design and manufacture, but in laymans terms, I wouldn't worry about it too much.
So long as your rifle shoots the bullets-loads you have chosen without displaying bullet instability, and you choose a moderator with enough clearance in the baffle bore,
then you should be absolutely fine with any moderator fitted.
rgds Pete