It looks(from your picture) like you're getting about the normal performance from the Mag-Tech 158g SP's. I doubt seriously if you're getting the advertised velocity from that revolver. I'd guess it's closer to 1150 fps at the muzzle. If you could shoot it over a chrono, you could know for sure.
Even though you're not seeing the "actual" degree of expansion when looking at those bullets, it's the "act" of expanding that plays a big part of terminal performance. As the nose expands, it transfers a lot of energy to the animals vitals. With no expansion, you're left with a very small frontal area, and more of a clean "practice arrow" type wound channel. Even though your SP's here don't look too impressive compared to the HP's, they've expanded to the point of having a larger than caliber frontal area, which will leave a lot more destructive wound channel aiding in a quick kill.
Don't get hung up on extreme expansion with the 357 magnum if you're intending on hunting deer sized game with it. The 357 shines best with a bullet weight/design, and velocity level that gives you "just enough" expansion to open the bullet noses up to caliber, or just over caliber sized on a solid facing shoulder hit, and still retain enough integrity/velocity to drive the remainder of the way through the animals chest, preferably exiting the off side. That Mag-Tech 158g SP ammo will serve you well as long as you make good clean vital chest shots on anything up to deer sized game.
