7.5 x 55 SWISS Subsonic

The 7.5×55 Swiss, originally designed for the Swiss military's Schmidt–Rubin rifles, is a highly accurate and well-balanced cartridge, though not typically associated with subsonic applications. However, with proper load development, it can be adapted effectively for subsonic shooting. The caliber's relatively large case capacity compared to the small powder charge required for subsonic velocity (below ~1,050 fps) introduces challenges, particularly in achieving consistent ignition and avoiding pressure spikes or squib loads. To safely and reliably reload subsonic 7.5×55 Swiss, experienced reloaders often use heavy-for-caliber bullets—typically 200 to 220 grains—and fast-burning powders such as Trail Boss, Unique, or Vihtavuori N310, which provide better case fill and stable pressure curves. 

Popular subsonic loads may use cast lead or coated bullets with reduced loads tailored for bolt-action rifles like the K31, which can cycle manually without issues. Key considerations include the risk of bullet lodging in the barrel if charges are too light, and the need for chrono testing and ladder loads to ensure both safety and accuracy. When done correctly, 7.5×55 subsonic loads can be very quiet when suppressed, offering a unique, precision shooting experience ideal for training or discreet target use. Always refer to trusted reloading manuals and start low, working up cautiously.