Hulls come in many gauges, colors, lengths, crimps and brass heights. While these differences are apparent from the outside, many hull differences are internal.
From Ballistic Products: Primer notice: Like Fiocchi, Rio's proprietary primer is made to a metric specification. The result is that the primer measures approximately 0.001" larger than standard U.S. primers. Therefore, plan on reloading them with Fiocchi or Rio primers- otherwise a U.S. primer will be loose in the primer pocket. Of course, primer pockets can always be repaired with a Primer Pocket Conditioner tool (sold separately).