Mike,
The OD at the muzzles is now 0.526" and the wall thickness, in the centre of the grooves, is now 0.081", plenty thick enough for the pressure out at that end. One thing that I learned years ago from Tony "Marrakai" is to keep the muzzles light and the weight at the rear of the barrels, if you want to end up with that lovely "between-the-hands" feel of the classic British double rifles. If you don't do this, you end up with a muzzle-heavy rifle that swings like a sledgehammer.

Best,
Ron.