Where are you hunting and what are the opportunity set? If Namibia, you will likely only have dik dik on the really tiny end of the spectrum. For common duiker and steenbok a rifle is a much better option. With a 300, I would use a super tough 200 gr bullet and try not to actually hit the shoulder. If the mountainous central part of the country, you may be hard pressed to get in shotgun range even of a dik dik.
Moz has blues and suni that could be hunted with a shotgun. As I noted above, I would not attempt to hunt Oribi, common, or reds with a shotgun.
I am sure your outfitter would have a recommendation.