In general, subsonic rifle ammo is very niche specific and rarely used. I can count on one hand the number of organizations I know of that have actually needed to use suppressed subsonic rifle projectiles in the real world. Subsonic rifle ammo is going to be below 1150 f/s...it then acts like a long handgun bullet and does NOT possess typical rifle projectile terminal effects. Also, think how the sound wave propagates away from the bullet flight path at supersonic velocities to see why the shooter location is hard to localize using a suppressor and supersonic rifle ammo.
For those who truly need subsonic or other similar specialty ammo, Engel Ballistic Research makes the best we've seen to date:
http://www.ebr-inc.net/Home.asp