flat out vs bluntly

flat out and bluntly both are adverbs.

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

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

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

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