Beretta Magazine Extension Pad w/Spring PX4 .40 S&W +3-Rounds Black 3. 40 S&W. Magazine Pad Extension plus spring increase the capacity of the 14 rounds magazine (.40 S&W) to 17 rounds. Specifications for Beretta PX4 .40 S&W +3 Rounds Magazine Extension Pad w/Spring: Manufacturer: Beretta Color: Black Fabric/Material: Polymer Magazine Capacity: 3 Round Caliber: .40 S&W Gun Make: Beretta Gun Model: Beretta PX4 Storm Package Contents: Beretta PX4 .40 S&W +3 Rounds Magazine Extension Pad w/Spring The Beretta PX4 .40 S&W 3-Round Magazine Extension Kit is a quality addition to the Beretta lineup. For more great deals on Pistol Magazine Extensions by Beretta please browse our Beretta Weapon Accessories page. What firearm models does the manufacturer say Beretta PX4 .40 S&W +3 Rounds Magazine Extension Pad w/Spring is compatible with? The manufacturer suggests that Beretta PX4 .40 S&W +3 Rounds Magazine Extension Pad w/Spring is compatible with Beretta PX4 StormBeretta E00344: Magazine Extension Pad w/Spring PX4 .40 S&W +3-Rounds Black