here is what works for me. 55gr flat base bullet(Speer-Sierra-etc.)TrailBoss powder; approx.3.5 to 3.8grs of powder. you may want to start at 4.0grs and work down to the 3.8gr load. TrailBoss nicely fills the brass almost half way. that helps to keep ignition consistent. standard small rifle primers work fine. do not go under the 3.5gr load as there is a fine line between subsonic quiet and BULLET STUCK IN BARREL! dont ask how i know. in my m-16 with HALO suppressor the loudest sounds are hammer impact and bullet impact. of course i have to manually operate the action. as always, use caution,my results may differ greatly from yours;different barrels,etc.