It happened around 8 p.m. Sunday in Limerick Twp., Pa.
Investigators say alert off-duty officer followed two suspects into the store who were dressed all in black and wearing medical gloves.
One of them pulled a gun on a cashier.
The officer pulled his own weapon and ordered the suspect to drop his gun.
The suspect complied and both men ran from the scene.
Officers tracked them down a short time later and arrested them.