Using Rem .40 brass and a lighter jacketed bullet in BHP 40 illustrates the reason for a u die. It ain't got nothing to do with "the bulge" from earlier glocks. For the 9mm, ,40 and 45 acp the u die provides for a greater interference fit if needed and expanding plug is suitably lessoned in dia. It can signifcantly reduce bullet setback. Bullet setback is much less of a concern with lower powered rounds using greater dia lead bullets and thicker brass.