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About the only reloading powder that I can think of that actually still accelerates the shot towards the end of a 30 inch barrel is black powder. That's why BP shells are so loud even in long barrels, there is still pressure in the barrel as the shot leaves the barrel. That's why it achieves the same velocity as smokeless with only a fraction of the peak pressure.
In general, the lower the peak pressure, the more the powder is using the whole barrel to accelerate the shot. Pick up a used shot cup and examine the shot dimples in the plastic. If you load with BP, the dimples are almost non-existant.
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