I've had a .300 Whisper, aka Blackout, since it's inception by my good friend JD Jones of SSK Industries. I've never tried these new subsonic rounds made by Hornady. JD had me test bullets in the Whisper to see what works. I was also hooked up with Lehigh Defense to help them see what bullet they were developing would work. I tried many of their early bullets, took pics of damage.
Bottom line on my results: "regular" bullets- the Whisper was devoloped with the 240/250 gr. Sierra HPBT Match. They killed by tumbling. Gave ragged exit holes with irregular internal damage. Deer would run. Groundhogs didn't get phased much. The Lehigh bullet that gave 95% instant kills & dropped deer was the 186 gr. Controlled Chaos- open nose, skived nose, .17 cal. bullet in the nose. A hit on deer gave 6 tracs of damage- 4 from nose pedals, one from .17 cal., one from base. Awesome bullet, but pricy.
I went on to test regular bullets. The winner was 220 gr. Sierra RN. Killed by expansion & tumbling. Not much of an exit hole, but good internal damage. On groundhogs, deer, turkeys, foxes, coyotes 95-98% instant drops!!! Reasonable price, as good as performance as the Lehigh. So far I've taken about 150 groundhogs, several turkeys & coyotes, 120+ deer including a heavy WV 8 pt. buck at 240 yds. in a driving rain/wind storm. He dropped!!
I've seen no reason to try the Horn. subsonics, but I'd be interested in hearing of performance on actual kills with it, especially at distance.
My Whisper is on a Contender frame. Saw no reason to go "automatic" since it's so quiet & I have plenty of time to reload.
A mildot scope is mandatory due to the great drop in trajectory at longer ranges.