The important thing here is we need to select the ows_contenttype crawled property as crawled property:
Also we need to make sure to check the Query Property while creating this managed property:
I have created my managed metadata with the name contenttypeproperty. Also lets asssume my content type name is democontenttype. Now we can do a search like
With this query the search will only return the items having the content type as democontenttype.
On the similar lines if we just need to return the documents in the search results we can change the query to:
With this query it will only be showing the documents in the search resutls core web part.