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Heavier rounds, such as HST 147, use longer bullets, which leaves less case capacity. The plusses are that pressure is achieved with less powder (due to pressure having an inverse ratio to case volume); the heavier bullet remains in the barrel longer (due to greater mass having more inertia and less acceleration); because of the previous two factors, a higher percentage of powder is burned while the bullet is in the barrel; because of that, the heavier round actually loses a lower percentage of its velocity out of a short barrel than does a lighter, faster round.
Many +P rounds have diminishing returns from short barrels as much of the powder is burned after the bullet has left the barrel. More velocity loss is induced, and more flash and bang are produced.
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