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The use of non-expanding bullets comes from the British use of the Dum-Dum bullets (named for the arsenal in India where they were manufactured) in the Zulu and Boer wars in South Africa. They tended to do massive damage when hitting non-critical areas of the body.
Now my quick thought on the variation in yaw performance based on identical guns (this may be covered somewhere else) is that it may be somehow related to the heating of the barrel changing the performance of the rounds as the barrel expands/contracts. If this changes the fit of the bullet, then you get less spin, more yaw. The tendency of a bullet to wander after impact is largely related to the lack of angular momentum gained by spinning the bullet (this effect is why rifling works). The more spin, bullet radius, or mass, the larger the angular momentum. Bullet weight matters because it both increases mass and decreases velocity (which reduces spin rate coming out of the same barrel).
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