Master gunsmith is a title Zimmerman earned for being in the business since as far back as he can remember, for serving three years in an Army ordnance unit and for graduating from the Colorado School of Trade, "the number one gunsmith school in the United States," he said.
Zimmerman spent a year studying in England at Holland and Holland, high-end gunsmiths who cater to discriminating gun enthusiasts, and worked for seven years for Griffin & Howe, a top-end American gunsmith, including a five-year apprenticeship there.
He worked in gun shops in Florida, Michigan and Waynesboro, Pa., before he took a job at Harpers Ferry National Historical Park in 1995.
Recalling how well things went when he set up a Civil War gun-repair tent during the 125th anniversary of the Battle of Gettysburg in 1997, he decided to buy the building in Harpers Ferry and specialize in repairing and restoring antique weapons.
His federal Bureau of Alcohol, Tobacco and Firearms license limits his shop to the repair and restoration of guns made no later than 1898.
Most of his customers are Civil War re-enactors who bring in their authentic weapons for repair. They make up about one-third of his business. Zimmerman also sells reproduction Civil War-era guns made in Italy. Re-enactors and black-powder deer hunters buy them.
Re-enactors rarely use authentic weapons.
"They're too valuable and historians object because weapons of such historical value could be damaged," Zimmerman said. "There are safety issues, too. The metal in old weapons deteriorates."
A half-dozen authentic early and mid-19th century rifles hang on a sale rack on the side of the counter in Zimmerman's shop. Their prices range upward from around $2,000.
On an opposite rack, a half-dozen or more reproduction rifles hang on another rack. Zimmerman removes their Italian markings and machine stamps them to show they were made in the Harpers Ferry Armory and the date they were made, a common practice for weapons used by re-enactors.
Nearly a dozen rifles and muskets lean against a floor rack in the back shop where the repair work is done. One, a 250-year-old Kentucky rifle, is in for restoration. The rest, mostly Civil-War-era weapons, are in for repairs.
Zimmerman uses original parts when he can. When real parts are not available, reproductions are easily obtained from many suppliers.
The oldest and most valuable weapon Zimmerman repaired was done while he was working for Griffin & Howe.
"It was a rifle made for Louis XIII of France in the 1600s," he said. The French king's gun is on display in New York's Metropolitan Museum of Art.
The room in the back is like any well-equipped machine shop with the exception of a couple of 19th-century lathes still in use alongside modern drill presses, steel rolling machines, lathes and grinders.
And, like any machine shop new or old, there are the machinist's most widely used hand tool -- files, dozens of them in all sizes, hanging in Zimmerman's shop.