I have no idea as to the why on the lead, but I suspect the reason you don't see loads below 8K is this powder will not burn properly at low pressures, "VERY FEW" slow powders will. Slow powders were basically not designed for low pressures, they were designed to move heavier pay loads, give higher velocities or Both. This was done by "Retarding their burn rates which is not normally compatible with low pressures
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Miller/TN
I Didn't Say Everything I Said, Yogi Berra