The rifles, including AK-47s and AR-10s, were discovered at the cargo terminal of Rio's international airport.
"They came from Miami in containers along with a shipment of swimming pool heaters," Rio police said in a tweet.
Pictures of the haul showed the weapons still lying in foam packaging and later lined up neatly along with their magazines.
The illegal cargo was estimated to have a value of 4.8 million reais ($1.5 million), the noticias.UOL news website reported. Four people were arrested in connected with the discovery, it said.
Rio's security forces have been unable to keep a lid on the flow of powerful automatic weapons used by narcotics gangs and shadowy militia groups which control many of the city's favelas.
Just this May, 32 assault rifles were captured when police responded to a clash between two rival drug factions in the favela known as Cidade Alta.
Shootings of gang suspects, police or innocent bystanders are an almost daily occurrence in Rio, where crime has soared since the end of last year's Olympic Games.
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