Yes, they are. There will be differences in headspace, end shake, firing pin length, and friction between various parts, all of which can act together to create tolerance stacking and result in misfires. You might measure the headspace of both your revolvers. I'd wager that you'll find that the headspace on the 686 is a thousandth or so more than the 66 has. Cylinder and Slide Shop makes an extra-length firing pin that would solve the problem, but you'll have to remember not to dry fire your gun if you install one, since the firing pin will eventually break if you do. I have their firing pins in both my 3" 681+PC guns, and they work well with even WW and CCI primers, with DA pulls of about 7 lbs on each gun.