I remember it too with NAP Thunderheads. Don't get me wrong they are great heads, but they are way heavier than they appear. I read (on here a while back) where someone weighed Thunderheads on a reloading scale. The 100 grain Thunderhead was actually 106, and the 125 grain was around 130.

I loved shooting Thunderheads, but always had to keep in mind, that 1/2" or so more drop compared to regular field tips and other broadheads. Like I said, Thunderheads are great, fly great, perform great, but they just fly a little lower than some.