But the 30-year-old's convictions were on Thursday overturned in the ACT Supreme Court by a "reluctant" Justice Michael Elkaim, who cited "fatal" errors in documents key to the Crown case.
His decision centred on several evidence certificates relied on by the prosecution, designed to prove that Clark was unauthorised to possess the weapons.
Justice Elkaim found there were errors on the certificates, critical to their admissibility in court.
For example, certificates for the knuckle duster and baton charges, referred to s17(a) of the Prohibited Weapons Act, and in fact should have read 17(1)(a). The document in effect referred to a section that "doesn't exist", the prosecutor conceded.
The magistrate corrected the certificate at the time, however, Justice Elkaim agreed with Clark's barrister Jack Pappas that this was beyond her power. The documents needed to be amended by the person who issued them, he said.
On the shotgun and ammunition charges, Justice Elkaim noted the relevant certificates needed to be signed by the firearms registrar, or a delegate of the registrar. The certificates in this case were signed by a police officer, and it was argued that there was no evidence of the officer's delegation.
During the appeal hearing, the prosecutor had conceded there "was not evidence" of the chain of authority, but if there were a transgression, it might be characterised as "accidental non-compliance."
Justice Elkaim replied, "But so what? We're talking about a criminal ... people are convicted of beyond reasonable doubt, not 'it seems like he is guilty'."
Justice Elkaim said the certificates were tendered as having been properly made and going "back through the chain of delegations and appointments" which ultimately permitted the officer to sign the document.
"The Crown now admits that the chain of authority did not exist. This must be fatal to the admissibility of [the documents]," he said.
Justice Elkaim noted the charges were serious, and that subject to the validity of the certificates, the case against Clark was very strong.
"I am, and should be, reluctant to set aside the convictions against this background," he said.
But he said a fresh hearing in the Magistrates Court would be pointless, because it would not cure the certificates' defects.
He set aside each of the four convictions.
"Although the appellant may be seen to be 'getting off on a technicality' it was not a technicality that became apparent on the appeal or was not stridently pursued before the magistrate."