I've done it on a couple Rem 700 243 Varmint barrels. I take the factory thread right off.
This is the best way to do it. /\
If you have a standard weight bbl, you may not get away with it.
Take off Varmint barrels, already chambered for the 6mm Rem or 243 can be had for as little as $150, if you do a bit of searching. One should screw right into your receiver and the other, if it's chambered for the 243 can easily have the tenon removed and recut.
As far as feeding, not a problem.
One thing you should realize, before going this route is that with 100 grain bullets, there will be little to no velocity gain over the 243
243 cases are much easier to locate.
You don't mention the twist rate of your present 243 barrel.
Early 6mm Remington barrels had 1-12 twist rates and seldom stabilize a bullet heavier than 100 grains. Later 6mm Rem barrels have 1-9 twist rates.
Most 243 Rem barrels have a 1-10 twist rate which should stabilize the heavy 6mm bullets.