In the early days of the 7mm Rem Mag, bullets were a bit fragile for the speed at which they traveled, leading to some less than desirable results. That may still be the case if you insist on shooting the cheapest amo you can find (read Remington CoreLokts). Now then, about this deer running away with half a head. Not calling you a liar, but it does sound far fetched. I have seen them run after some jackass blows off a lower jaw or a nose by trying a headshot.