That load of 4.8 grains Trailboss under the XTP is incredibly quiet, I shot some more of them this past weekend. They do not feed from a magazine though. I think it's because the OAL is too short, and the HP just deforms if it gets slammed into the feed ramps. I have to push them into the chamber with my finger. I put a picture in another thread, but I guess I'll add it here too:
That one was run at 1900 fps into gallon water jugs. It blew up the first one, and stayed inside the second one. I haven't shot a water jug at subsonic velocities yet, but I think they will expand quite nicely. Someone should try it when you get some in.
-Dan