Comparison of Bush era gunwalker with Obama era gunwalker
Story here. Bottom line: the 2006 Operation Operation Wide Receiver tried to use radio locator devices in the rifles, tracked by aircraft. They found the plan didn't work. The antennae of the radio locators had to be folded up to fit, shortening their range, the smugglers ate up time until the aircraft had to refuel. Upon discovering that it wasn't working, the Operation was canceled in 2007.
There are still some unanswered questions such as why the transmitters and couldn't have been modified with more space for the antenna a batteries, and the leaks don't tell us whether the guns would have been interdicted. But the bottom line was the operation was canceled, leaving the question of why Operation Fast and Furious was begun a few years later, sans (with a few exceptions) radio locators.