Outlaw's Gun by J.R. Roberts

Clint is tracked down by an outlaw who, in his fifties, finds that his gun is slowing down. He wants Clint to use his gunsmithing abilities to make him a new gun, one that will increase his speed and, hopefully, shave a few years off his gun hand. Months later, Clint learns he's been made a fool of. The outlaw is using his newfound speed and accuracy with a gun to settle some old scores—and some new ones. It falls to Clint to stop him, and to see if the outlaw's abilities with a gun match his own.