I had a halo of tiny little holes show up around the primary holes in my target one day at the range ... but that was from pushing 55gr JSP's beyond 4000 fps
... I figured that my lead tips were melting. But out of a pistol, the only thing I've ever seen is when you push plated bullets WAY too hard (>>1350 fps) and the plating shears off ... if you are using plated bullets and the barrel is stressing them somehow (rough or too tight???) then maybe that is the cause ... personally, if this is happening regularly, I'd be very concerned that something very wrong was happening.
Be safe,
Saands