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Hi Craig and welcome. Small pistol primers as shown in the 2nd picture here (just change the 9 to a c after the www.) : https://www.9heaperthandirt.com/underwood-ammo-.357-magnum-ammunition-20-rounds-xtp-jhp-158-gr/fc-816874020446.html --------- Cheers and stay safe!

Hi Craig, Underwood XTP Ammo in .357 Magnum, specifically the 158 Grain variant, typically uses Small Pistol Magnum Primers. Using small pistol magnum primers is crucial for achieving optimal performance with this ammunition. They help ensure reliable ignition and consistent performance, especially under varying conditions. Stay safe & have a great day!

They use small pistol primers 🎯❗️

Small pistol primers.

Underwood Ammunition utilizes Small Pistol Magnum primers for their .357 Magnum loads.Because they load their ammunition to maximum velocities (often using slow-burning ball powders to achieve top-tier energy), magnum primers are standard to ensure consistent and complete powder ignition.Important Note for Reloaders:If you are asking this question to reload or duplicate Underwood's high-performance ammunition, be aware that you cannot swap magnum and standard primers interchangeably without altering the ballistic profile. Always follow published load data using the exact components listed by the powder manufacturer.

Underwood Ammunition utilizes Small Pistol Magnum primers for their .357 Magnum loads. Because Underwood loads their ammunition to maximum, full-power pressures (often using slow-burning, flash-suppressed ball powders like H110 or W296), a magnum-strength primer is required to ensure consistent and complete ignition of the powder.