Martin-

There should be a machine shop in your area.
Take your barrels to the machinist and have them lightly counterbore (recessed crown) the muzzles of your rifle - both the same bore and depth, to slightly greater than groove diameter.

If you look closely, you'll be able to see this has been done to the muzzles of this rifle...

Make sure that they carefully deburr the job, I typically turn a brass or aluminum rod to similar diameter and contour as the crowns and lap them clean with valve grinding compound.

You should be able to get it turned around inside one day.

--Tinker