Herb, I haven't done a muzzle like that on a Hawken yet, though I know it was a common crown system then. I did a similar job on the muzzle of my Virginia rifle though. The photos are reversed...the bottom one is the 'before' shot , and is my standard way of crowning a muzzleloading barrel. This is a round bottomed Rice .50 cal barrel. Just for fun, and to see what would happen to accuracy, I filed the grooves with a round needle file - the top picture. Accuracy did not diminish in the least. As you can see, I only filed in about 1/8", which is what I observed in most of the Jaeger rifles in Steinschloss Jaegerbuschsen. A lot of the muzzles on original Hawken rifles have this treatment, but the photography is never good enough to see exactly how far they've filed.
May I suggest that it is necessary to first give the muzzle a slight radius to break the edge of the lands, and polish it. Then you can deepen the grooves. If you don't first polish the end of the muzzle, you will most certainly cut patches upon loading, that is unjless you're using a patch/ball combo that is ridiculously loose, in which case you won't have any accuracy anyway, and unacceptable fouling accumulation.
Jim Gordon's book #3 has many examples of this type of muzzle treatment.