"I believe this election is forcing me to choose between two unconstitutional methods of execution, and I do not intend to waive any challenges to electrocution or firing squad by making an election," Moore said in the statement.
Moore's attorneys have asked the state Supreme Court to delay his death while another court determines if either available method is cruel and unusual punishment. The attorneys argue prisons officials aren't trying hard enough to get the lethal injection drugs, instead of forcing prisoners to choose between two more barbaric methods.
His lawyers are also asking the state Supreme Court to delay the execution so the US Supreme Court can review whether his death sentence was a disproportionate punishment compared with similar crimes. The state justices denied a similar appeal last week.
South Carolina is one of eight states to still use the electric chair and one of four to allow a firing squad, according to the Washington-based nonprofit Death Penalty Information Centre.
Only three executions in the US have been carried out by firing squad since 1976, according to the nonprofit. Moore's would mark the first since Ronnie Lee Gardner's 2010 execution by a five-person firing squad in Utah.
South Carolina's corrections agency said last month it finished developing protocols for firing squad executions and completed $53,600 in renovations on the death chamber in Columbia, installing a metal chair with restraints that face a wall with a rectangular opening 4.6m away. In the case of a firing squad execution, three volunteer prison workers will train their rifles on the condemned prisoner's heart.
Moore is one of 35 men on South Carolina's death row. The state last scheduled an execution for Moore in 2020, which was then delayed after prison officials said they couldn't obtain lethal injection drugs.
Corrections Department director Bryan Stirling reiterated in an affidavit last week the agency still couldn't obtain the drugs because manufacturers and compounding pharmacies contacted by the state refused to help.
During Moore's 2001 trial, prosecutors said Moore entered the store looking for money to support his cocaine habit and got into a dispute with Mahoney, who drew a pistol that Moore wrestled away from him.
Mahoney pulled a second gun, and a gunfight ensued. Mahoney shot Moore in the arm, and Moore shot Mahoney in the chest. Prosecutors said Moore left a trail of blood through the store as he looked for cash, stepping twice over Mahoney.
At the time, Moore claimed he acted in self-defence after Mahoney drew the first gun.
Moore's supporters have argued his crime doesn't rise to the level of a death penalty offence. His appeals lawyers have said because Moore didn't bring a gun into the store, he couldn't have intended to kill someone when he walked in.
The last person executed in South Carolina was Jeffrey Motts, who was on death row for strangling a cellmate while serving a life sentence for another murder.