MOULD comes from England, like COLOUR instead of COLOR, FAVOURITE instead of FAVORITE, and HONOUR instead of HONOR. MOULD is traditional and since most shooters are traditionalists, it is the more common spelling in the shooting world.
MOLD is the correct American spelling and the more common spelling when talking about molding anything other than bullets. For example, a google for "plastic MOLD" turned up 165,000 results vs. 136,000 results for "plastic MOULD" (mostly from Asia). "MOLD maker" turned up 77,900 results vs. 33,400 for "MOULD maker."
The American plastic industry uses MOLD almost exclusively. The plastics trade magazine that I receive never uses MOULD.
The Random House dictionary confirms that MOLD is the correct spelling and that MOULD is British.
Since I am American and not English, and I am modern and not a traditionalist, I prefer MOLD.