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Not to sound disbelieving, but am I understanding correctly that the rifles shoot the same calibers as the semi-automatic handguns? Is there any meaningful difference between the velocity of a same-caliber bullet shot out of a different gun?
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Yes and no. It depends on several variables. This is a slight sidetrack, but I'll try to keep this brief.
As powder burns, it creates a column of expanding hot gas that pushes the bullet down the barrel. Think of it as an expanding cylinder; as the bullet moves down the barrel, the cylinder gets taller.
When the powder is completely burned, the pressure of the column of hot gas drops off sharply. From that point, the bullet is slowed down by friction between the surface of the bullet and the inside of the barrel. Therefore, there is an optimal barrel length for each cartridge, one that gives the gas enough room to expand, but no extra room to cause the bullet velocity to decline due to friction. This optimal length is determined by the type of powder, the shape of the bullet, and the capacity and shape of the cartridge case.
Pistol cartridges typically have small cases filled with fast-burning powder pushing a relatively large-diameter bullet. The diameter is important because greater diameter equals more surface area rubbing against the barrel, and therefore more friction. The small case and fast-burning powder will accelerate the bullet quickly, but the gas will be expended quickly, and velocity will drop off if the barrel is too long.
Conversely, most modern rifles use a large case filled with slow-burning powder and a relatively small-diameter bullet. This type of cartridge needs a longer barrel to give the gas enough room to expand, but yields a much higher ultimate velocity. If the barrel is too short, the excess hot gas will wastefully blow out the end of the barrel, creating lots of muzzle flash and contributing little to the velocity of the bullet. (Ask anyone who has ever fired a lightweight short-barreled carbine in a high-powered rifle caliber. Think "fireball".
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Most pistol-caliber carbines will fire bullets at somewhat higher velocities than the same load in a handgun, but the difference can be small, and such carbines are usually offered only in relatively high-powered, high-velocity pistol calibers (like .44 Magnum) with relatively short barrels by rifle standards (16"-18"). This minimizes the theoretical performance disadvantages.
OK, back on topic now.
