We spotted Phil Mickelson deviating from his traditional Project X HZRDUS T1100 on the range ahead of the Wells Fargo Championship. In its place, Lefty was testing a prototype Aldila Rogue Black 130 M.S.I. 60-TX shaft in his Callaway Epic Flash Sub Zero (3 diamond head, 10 degrees, set at neutral). The shaft is 45 inches and tipped one inch.

Rather than speculate as to why the left-hander is testing other shaft waters, we have word straight from Callaway’s tour truck.

Reportedly, Mickelson tested the shaft at the Ely Callaway Performance Center and liked it, according to Callaway. The main reason for this? He’s able to square the face easier.

Word from the truck is the shaft cuts down on Mickelson’s traditional miss, “a hanging ball to the left.” Beyond that, launch, spin, and ball speed are all similar to what he was seeing in the T1100, thus it only makes sense for him to give it a go.

Mickelson has bounced back and forth between driver heads this year (Rogue Sub Zero, Epic Sub Zero), but this is the first shaft change we’ve seen from him this season.

Certainly, the pursuit of accuracy makes sense for Mickelson, who currently sits at 208th in driving accuracy, hitting just 51.07 percent of fairways. He’s presently 68th in strokes gained: off-the-tee, however, the function of his impressive distance this season (304 yards on average, 27th on tour). If he’s able to find a few more fairways, the big stick should be a big weapon for the Phil Kwon Do calf-training master.

• All our photos from the Wells Fargo Championship, plus discussion here.