Scorching on exterior mouth of Win. LB hulls?

[decoykrvr]

A friend ,who like myself, has evaluated the new Win LB in both 3"and 31/2" w/ a variety of guns and choke configurations, and I were discussing the Win. LB shell design, performance and developmental potential the other day.  I had commented that if the shell could be upgraded to  @ 1300 fps it would equal or exceed the energy of some of the EnvironMetal Hevi-13 loads.  My friend commented, " I don't think that may be possible, because, have you noticed the scorching on the exterior of the mouth of the shells which extends back up the hull to an inch or more?"  Any thoughts and or ideas about this phenomenon: pressures, set-back, powders, design etc. The only shotgun hulls on which I have observed similar scorching were from slug loads, primarily w/ sabots.  As an aside, I'm not trying to stir the pot. In over 40 years of lead shell loading, experimentation, and evaluation I've never seen a lead load perform like the Win. LB.

[shadetree callers]

I suspect it may have to do with the new crimp . Sabots also have a rolled crimp but are open in the center .