flat out vs bluffly

flat out and bluffly both are adverbs.

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

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

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

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