Here are a couple of pictures of a .550" barrel (HS Precision 300 short mag).
This brake is .700 in diameter. I don't like them any smaller although it could be turned down to match the barrel contour... I think the larger diameter works a bit better.
Rather than try and take it right down to the barrel contour I prefer to stay a few thou larger and finish with a small radius.
Blasted for a dull finish.