Brass cracking just behind shoulder

[RobRat]

I'm on my 5th loading of a batch of 300/221 brass, and am finding cracks just behind the shoulder on a number of the cases. The cracks run about 1/2 way around the circumference of the case, on the straight part, at the edge of the shoulder, just past the radius.

I started with Lake City 07 5.56 brass, cut them off just behind the shoulder with a Harbor Freight mini chop saw, formed into 300/221 with a Reading sizing die, then final trimmed to length.

The rifle is an AR-15 upper, that I had a gunsmith friend of mine build for for me.

I thought that I might be bumping the shoulder excessively, and am finding that I'm bumping the shoulder about 0.004", which I'm thinking isn't excessive.

I did not anneal after forming- -could this be the cause? would this help?

Is there better brass to use to form the cases with?