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While the general stopping power of a pistol is measurably low compared to almost any long gun, you want to make an informed choice to give you the best advantage.  Smaller calibers, such as .380, 9mm, etc, generally result in a higher bullet velocity, reduced recoil which allows for fast follow-up shots, higher capacity magazines (or smaller framed guns), and higher velocity bullets.     {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/f\/ff\/Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet1.jpg\/v4-460px-Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet1.jpg","bigUrl":"\/images\/thumb\/f\/ff\/Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet1.jpg\/aid139610-v4-728px-Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet1.jpg","smallWidth":460,"smallHeight":345,"bigWidth":"728","bigHeight":"546","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}  Large calibers, such as the venerable .45 feature slower bullets that have increased overall diameter, allowing for a wider initial wound channel (which gives you a slightly increased chance of hitting something vital). The heavier bullets and generally high muzzle energies increase recoil, resulting in slower follow-up shots and the larger diameter of the cartridges means lower capacity magazines.     {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/f\/f9\/Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet2.jpg\/v4-460px-Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet2.jpg","bigUrl":"\/images\/thumb\/f\/f9\/Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet2.jpg\/aid139610-v4-728px-Choose-a-Firearm-for-Personal-or-Home-Defense-Step-4Bullet2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":"728","bigHeight":"546","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"} Most U.S. self-defense experts consider .380 ACP (9 x 17 mm Browning Short) or 9mm as the minimum for adequate defense, while European Police believe that .32 ACP (7.65 x 17 mm Browning) as adequate. Making an informed decision about what caliber you will use requires experience.  Shoot many different calibers and choose the biggest caliber that you can comfortably shoot and make fast, accurate followup shots with. A semi-automatic .22 pistol or Ruger 10/22 carbine with a high-capacity magazine is easy to control when shooting fast and can be very effective at close range (most home-defense shots occur at less than 7 yards), and .22 ammunition is very cheap. Furthermore, a .22 won't penetrate as many walls, thus reducing property damage and risk to your neighbors if you miss. You'll want to be able to easily afford enough ammunition to keep up a regular practice routine.
Caliber selection in pistols is a hot source of debate amongst tactics-minded handgun users. The most crucial factor in determining what caliber you will choose is how accurate and comfortable you are with any particular caliber. Don't completely disregard a .22 if your objective is home defense. When making your decision, also keep in mind the cost of ammunition!