Complex sentence: It was possible to lower the Whitney hammer between cylinder chambers for safe carry, but without the Remington milled slot the Whitney cylinder could possibly slip and rotate, allowing the hammer to strike a loaded, capped chamber and cause an accidental discharge.
Split into simpler sentences: It was possible to lower the Whitney hammer between cylinder chambers for safe carry. However, lacking the Remington milled slot, the Whitney cylinder could possibly slip and rotate, allowing the hammer to strike a loaded, capped chamber, causing an accidental discharge.