A lot of it can be the result of the pressure curve.
Black powder burns at a constant rate, and rather quickly. All of its push is done early in the bullet's travel down the barrel.
Smokeless powder, however, being progressive burning, has a much longer burn time, and depending on the initial chamber pressure, much different burn characteristics.
Smokeless powder will have a much longer pressure curve, meaning that it will keep pushing on the bullet for a much longer time.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
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