Personally i would have gone with a Bolt-rifle with a scope or a powerful las-gun. or something along the lines of the Vendacair rifle. 

And, im not sure if a ratling could carry an autocannon, unless it was gutted to remove major components like the auto-loader for magazines and most of the cooling systems. Personally i expect it would be a bolt action single shot weapon, that would actually be quite versitile against any infantry and some vehicles. But then i would go with a Las-cannon for the same effect and not have to carry around the presumably  heavy darts. Although if i find another game where i might play an Astartes ill keep this in mind :brow