The 17-year-old, from Rugby, Warwickshire, has been found guilty of preparing for "acts of neo-Nazi terrorism".
Prosecutor Matthew Brook said evidence showed the teenager wanted to create a firearm capable of "smashing heads" after joining a patriotic group. The teenager said he had not intended any act of terrorism.
Judge Paul Farrer QC remanded the teenager in custody until a sentencing hearing on 6 November.
He had previously pleaded guilty to nine charges in relation to "possessing documents likely to be useful to a terrorist".
Jurors at Birmingham Crown Court deliberated for more than 15 hours over four days before unanimously convicting the teenager, who was 16 at the time of the offences, of "preparing for terrorist acts" between April and September last year.
At the start of what was a re-trial, Brook said the boy had praised Brenton Tarran who carried out a mass shooting last year in Christchurch, New Zealand, killing 51 Moslems at two mosques.
"He came to believe an ideology which thinks a race war is coming, an ideology which believes its followers should bring about a race war, should accelerate its start, so that the white race can become supreme," he said.
"He came to believe in an ideology which praises terrorists who carry out mass shootings, like the Christchurch shootings in New Zealand, and called the perpetrators of such terrorist massacres 'saints'."
The court was told the boy had researched how to convert a blank-firing gun, offered advice to members of National Socialist chat groups and was admitted to an online NS group after completing a test survey in which he expressed a hatred for Jews.
Following his arrest last September, it emerged he had asked an adult friend for advice on where he could buy a blank-firing gun.
Gun-making instructions were found on his phone, and knives and a home-made gun stock were seized by police from his bedroom.
Brook said the boy claimed, although he had been discussing converting guns, it had in fact all been a fantasy and he had not done anything in the real world.
Head of West Midlands Counter Terrorism Unit Det Ch Supt Kenny Bell, said: "This boy had an unhealthy interest in other attacks across the world and he knew exactly what online platforms to join to share his extreme views."