Sierra jackets, eh - I was taking into consideration the angled cuts, Tinker. I have done the same on a belt sander. On some I cut on the middle line, I've reached into/onto the sides with a lathe chamfer tool and removed the "flashing" for a truer picture of the jacket thickness.
Others yet, I've scrunched them slightly in the vice lengthwise to expand the jacket away from the lead, then removed the flashing to measure jacket thickness. It was quicker than melting the cores out.
With the pure lead core and the normally 'soft' Sierra jackets, you should see expansion. Good choice of components.