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Old January 18, 2013, 02:28 PM   #32
Rainbow Demon
Senior Member
Join Date: September 27, 2012
Posts: 397
I am not 100% sure why the semi-auto Pistol caliber carbines dont gain near as much velocity as the revolver caliber carbines. Though a 10mm Carbine would work pretty well in my book.
Its a matter of the type of powder and size of powder charge normally used.
A revolver cartridge fired in a revolver loses some velocity due to gas escape at the cylinder gap. When that gap is eliminated theres an immediate benefit. Then add that the otherwise wasted gas will continue to expand in the increased length of bullet travel.

Autopistol cartridges are loaded with efficiency in short barrels in mind, and generally smaller case capacity which means more powder is burned in the first few inches of bullet travel, with coresponding increased chamber pressure compared to similar sized revolver cartridges. A loose comparasion with be the .38 S&W compared to the 9mm Luger.
Generally the auto pistol will use lighter bullets, which means faster acceleration in the shorter bullet travel.

In short the autoloader cartridges are more efficient in the shorter barrels.
The revolver cartridge is not used to full efficiency in a short barrel with cylinder gap.
Add to that heavier bullets making better use of increased length of bullet travel.

Early 9mm SMG specific loads sometimes used slower burning powders to take advantage of the increased bullet travel of the longer SMG barrels of the day, this allowed significant increases in velocity.
A modern 9mm load , or handload, using slower powders would produce a significant increase in velocity from the carbine length barrels.
Such a load would not be efficient in a normal length pistol barrel. Increased muzzle flash and reduced velocity would be the result.
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