Pointless and dangerous exercise, once you get the round to go slow enough it's a real liability. No killing power and when you have one stuck up the spout unawares and follow up with a second, perhaps full power, you'll see what I mean. Free piercings all round.
Sub sonic rounds are available in .22lr and in 9mm para. I use little else in my .22lr (Remington hollow point) and it's by far the biggest killer in my gun cabinet. I've culled park bred fallow with it very effectively, head shots, no pack drill.