In shooting my Modl 60 in double action mode I found that primer mfg does make a difference. CCI primers are harder (or need to be hit harder) that the older, silver winchester small pistol primers. When I say harder, I mean that the modl 60 wouldn't always hit the primer hard enough to ignite it when I shot double action (just pulling the trigger without cocking). Don't know if the outside metal on the primer is harder or not.
Another view, of the same question. Is it the fact that the Modl 60 doesn't compress the hammer spring as hard in double action as it does in single action? Thats why double action mode on the old Smiths was so slick.
In single action mode the Modl 60 would shoot both the CCi primers and Winchester primers just fine. The hammer spring is compressed more in single action mode.