Gunsil is correct... by definition, a Minie-bullet must have a cavity in its base, and speaking more specifically, that cavity must be large enough to help the base expand outward into the gun's rifling-grooves.
Unfortunately, your bullet does not appear in any of the various civil war bullet reference books I own, nor have I ever seen your bullet's form before. It is "close" to a super-rare variety of .52-caliber Sharps bullet... but your diameter, length, & weight measurements prove it is not that Sharps bullet (see "Handbook of Civil War Bullets & Cartridges" bullet #116).