flat out vs roundly

flat out and roundly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
flat out	No	No	No	Yes
roundly	No	No	No	Yes

As adverbs, flat out and roundly are synonyms defined as:

Other synonyms of flat out include bluffly, bluntly, brusquely.

flat out (adverb)	roundly (adverb)
in a blunt direct manner	in a round manner
at top speed	in a blunt direct manner