Careful about the efficiencies, they can get called out in different ways, and especially can be misleading when comparing different types of mills.
My most comprehensive literature is twenty years old:
N.L. Weiss, 1985, SME Mineral Processing Handbook, Vol. 1,ISBN 0-89520-433-6.
Browse through Sections 3A (General Aspects of Comminution) and 3C (Grinding).
SME (now Society of Mining, Metallurgy, and Exploration) can be located most readily at www.smenet.org
Anyway, SAG mills can be calculated as inefficient, except they do not require a large grinding load like ball and rod mills. Capital and operating costs have to be compared, system vs. system in order to assist with the selection. The feed suitability vis-a-vis the grind method is critical.
As far as the balls, and rods for that matter - wear hardening is a fascinating thing. Yeah, they can blow up.
Ruble3, nice to see you again. I'm bemused that it was an environmental manager who smoked his office. The milling operations I am aware of have been careful about allowing used balls out as souvenirs. Sometimes that spalling, especially on old cast balls, can get fairly energetic.
Have a safe shift.