The bore diameter in the muzzle brake for a 6mm should be ..264
.250 is two tight. the bore diameter is only .014 thousandths larger than the bullet. Any error in alignment could easily cause bullet strike, ruining accuracy.
It is best to bore the brake on a lathe, but if you already have a .250 bore in the brake you can drill it out if you are careful to get it aligned.
If you decide to drill it, I would recommend a .270 bore for clearance error of the drill bit walking off center.
.020 Is the recommended clearance but .025 will be safer and nor take away from the brakes efficiency.
If you have a smith install it, have him bore it on his lathe to the proper diameter.