Ring Bulges are normally the result of a local "Pressure Spike" which has absolutely nothing to do with the max or chamber pressure of the load. It is brought on by the charge being "Checked" in its travel down the bore. This could be from an obstruction or from large shot bridging in the choke.
One other phenomenon which has been cited as a possibility of a ring bulge is improper ignition resulting in just enough force to drive the charge a bit down the bore & then the main charge igniting & catching up in which case the projectile itself acts as the obstruction. This of course would not likely be the cause of a bulge at the muzzle, but would occur much earlier in the travel down the bore.
A great reason for not mixing mild primers with low pressure loads of slow powders.
