A discarded balaclava, CCTV footage, and evidence from the accused's sister helped a jury find Dylan Elias Keenan, 32, guilty of holding up the supermarket with a shortened firearm.

The ACT Supreme Court jury found Keenan guilty late on Thursday, following a trial before Justice John Burns.

The verdicts came a day after Keenan had a significant struggle with corrective services guards in the courtroom, after he said he suffered a panic attack.

Security was beefed up around Keenan on Thursday, and he was kept in handcuffs in front of the jury.

Keenan stormed the supermarket on August 19, bursting in and striking a staff member in the face with the gun.

He demanded cash from another staff member, and about $1500 was put in to a supermarket shopping bag before Keenan fled.

"I was just looking at this barrel and I was thinking 'Please don't pull the trigger'," the staff member said earlier in the trial.

CCTV footage captured part of the crime.

Keenan fought the charges, arguing he wasn't the man pictured in the vision.

But a balaclava was later found in a nearby carpark and it was linked with Keenan's DNA.

His home was raided and a gun similar to the offending weapon was found, as were a pair of gloves and a jumper the Crown alleged were similar to those used.

A cartridge of a similar type to one seemingly missing from Keenan's gun was also found at the scene.

His sister gave evidence that Keenan had asked her to bury his gun in her backyard.

Another witness saw, before the robbery, a car similar to the one Keenan owned turning into the Chapman Primary School car park, where the balaclava was found.

The jury took less than an hour to find Keenan, of Cooma, guilty of aggravated robbery.

Earlier in the day, Keenan gave evidence, claiming he was not the man responsible for the crime.

He said the discovery of the cartridge at the scene was a coincidence, but had nothing to say about the balaclava.

His sentencing will begin in November.