Soft lead for 3 reasons in a rifle...
1)When you load the ball, the soft lead will actually swage itself to the bore of the rifle, that's why it might be harder to load the first 6 inches or so, then slides down easier...Also, load a ball sometimes, push down to the breach and then pull the ball...You will notice the "hatch" pattern of the patch embedded into the ball, less so on the grooves than on the lands...
2)Upon firing a ball "squats"...It actually gets fatter because of the rapid increase in pressure, this helps to seal the gasses, giving better velocity...
3)When the ball hits the target (deer)....A pure lead ball tends to flatten faster than a harder ball, transferring energy faster to the animal...In addition, pure lead has a tendence to hold together better, so you retain more weight, increasing penetration...