I just took a few pics of the Zero bullets to post here. Looking at them, the distance from the base of the bullet to the crimp groove/serrations is very nearly the same on both bullets, so case volume differences with the bullets in the loaded condition are minimal, but might help account for the differences in velocity. The JHP bullet is longer, .650 inches versus .635 inches for the JSP bullet. Both bullets show a diameter of .357 according to my dial calipers. I'm thinking the velocity differences between them might also be tied into the bullet configuration above the crimp groove more than anything else, but that is just a guess. BTW, with revolver loads, I load to the crimp groove and not OAL.

Here are the pics. The JSP bullet is the one in the middle and the JHP bullets are on each side of it.



RW, the velocities might seem low at first glance, but you are comparing apples and pears as far as barrel length goes. My 627 Pro has a 4" barrel. Looking at the Alliant site, they are showing a max load of 14.8 grains of 2400 for 1265 fps, but that is with a 10" barrel. That 6" of extra barrel length can account for the different velocities.