หามาให้นะครับ เอามาจาก วิกิพีเดีย
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Overpressure ammunition, commonly designated as +P or +P+, is small arms ammunition that has been loaded to a higher internal pressure than is standard for ammunition of its caliber (see internal ballistics), but less than the pressures generated by a proof round. This is done typically to produce rounds with a higher muzzle velocity and stopping power, such as ammunition used for defensive purposes. Because of this, +P ammunition is typically found in handgun calibers which might be used for defensive purposes.
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และความเข้าใจของผมเอง ตัว P นี้ น่าจะเกี่ยวข้องกับคำว่า "Pressure" หรือ แรงดันครับ
