Thread: See separation?

Not far enough annealing past shoulder, about 5mm needed otherwise you had a hard brass to soft brass junction at neck and shoulder junction which is most likely the highest stretching/stress region on the cartridge when fired and causing case not expand uniformly in this region.

black stuff - just carbon as stated earlier, can darken more with gas annealing

Rapid cooling of brass is safe as houses and does not harden it.

One can tell if you over annealed brass with gas, first you can see the flame slightly change colour as the surface zinc in the brass burns out of the copper, also the more than usual shinny tin look to brass once annealed - not the usual annealed colour. Anyway I have fired over annealed brass as fouler shots before range shooting and never had a case separate nor any other problems apart from maybe a dented shoulder when seating projectile (but they still fire form out and case reharden with use)