The 240 XTP is a .429" bullet, while the 240 grain XTP-Mag is a .452" bullet. Hornady does not make an XTP-Mag in .429" caliber, nor do they make a standard 240 grain XTP in .452" caliber.

The regular XTPs were designed for the more subdued velocities you might get out of a 44 Magnum or 45 Colt revolver, which is to say 800-1400 fps. The XTP-Mags were designed for relatively high velocity cartridges such as the 454 Casull and 460 S&W Magnum, which can produce velocities of 1800-2400 fps.

Being quite familiar with both bullets, I took a look at the bullets loaded in the Hornady packages at my local store. In one of the Hornady XTP/sabot boxes, specifically the one labeled 50 cal, 240 grain with the green sabots, those bullets are the .429" regular XTP. In the box labeled 50 cal, 300 grain Mag-Express with the black sabots, the bullets were unmistakably .452" XTP-Mags, even though the bullets aren't specifically labeled as XTP-Mags.