I just talked to the tech department at Hornady. A very nice gentleman took the time to explain all of this to me. Their plated bullets are made to the exact groove to groove diameter. In a perfectly made barrel with a perfectly made plated bullet, there will be zero clearance and zero engagement with the barrel groove surface, just sliding friction. The lands will dig into the bullet and force it to spin. Obviously there is a lot of friction drag associated with that. The bullet will obtruate some, but it is not necessary or designed for. The plated bullet is manufactured to groove to groove diameter plus or minus 1 or 2 ten thousands (he wasn't sure). (I suspect that that is S&W's tolerance on bore and groove diameter also).

In the case of their lead bullets (swaged), for the .357 they are made .001" over to take advantage of better sealing due to the interference fit with the groove surface. THe lead is soft enough to compress the diameter to the groove diameter. Also, obtruation definitely occurs adding to the sealing effect. The down side is that their relatively soft lead bullets will lead (smear)the barrel if driven much over 1000 fps. They get good efficiency for target work (less powder, less recoil) because of the good sealing, but are not good for the higher velocities needed for hunting. As you add hardening materials to the lead, there is less leading at the higher velocities, but efficiency is lost due to less sealing effect. At some point the hardened lead bullet starts to act like a plated bullet. Of course a lot of engineering goes into the terminal ballistics, and I am sure that effects the materials used and how they will behave in the barrel. Hope this helps those who want to learn why things are the way they are. To me, it makes reloading and shooting more fun.

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686 & 629