According to Ned Roberts in the Muzzleloading Caplock Rifle, the old-timers PREFERRED " weak" caps for target shooting, because they gave better accuracy. I have a primer adapter that allows you to shoot a caplock using small rifle primers instead of caps. One time I was doing some chronographing of a Maxi-ball load in a .45 rifle, and with the primers, extreme velocity variation exceeded 100 FPS! I installed a regular 1/4X28 nipple, and started using CCI No. 11 caps. Extreme variation dropped to 11 FPS with the SAME powder charge and bullet!! This seems to support what you discovered.