Elvis,
When the Grand Slams first appeared in the 1970's they were pretty soft, probably because the jackets were a little thin. I know this from personal experience on big game.
After a few years they got tougher, and generally acted much like Nosler Partitions on game. They could be pretty damn accurate as well.
When the dual-core went away, they became much like thicker-jacketed Hot-Cors, similar to the Mag-Tips. I first tested some 200-grain .30's from a .300 Winchester Magnum at 2900+ fps, shot into dry newspaper at around 30-35 yards, my standard "tough" test for big game bullets, since it's similar to hitting heavy bone. I shot five into one stack of paper, spacing them so they each went into fresh paper. Three of the five retained their cores and, as I recall, around 70% of their weight. The other two lost their cores, so didn't penetrate very deeply.
