A detective said she believed the teenager’s intention was to intimidate another person after using the “dark web” in a bid to obtain the lethal haul.
The child was detained in a police operation at the Riverside retail park, in Coleraine, Northern Ireland, on Thursday morning.
He had allegedly handed over £150 for the ammunition to an operative, police said, believing he could further purchase a machine gun.
The offences are alleged to have occurred between March 27 and April 7 this year.
Covert evidence had been obtained and police said they were downloading his online interactions.
The accused, wearing a grey hoodie and red T-shirt, appeared at Ballymena Magistrates’ Court flanked by his father and three police officers.