[LouBoyd]

The most important cons are:
1. The 6.8 SPC has considerably more case capacity than the 300-221, yet it would be shooting lighter bullets. It will be difficult to get good subsonic velocity consistency without filling the cases to reduce the capacity.
2. The case is too long to properly seat heavy low drag .277 bullets and still fit in an AR-15 magazine. There's not much point in an AR-15 single shot.
3. The lighter subsonic bullets won't deliver as much downrange energy as the 300-221. Even the 300-221 is marginal for hunting deer. The only way to get decent downrange energy from any subsonic is to use both heavy and low drag bullets. That's why the 300 Whisper, 338 Whisper, 458 Socom AR-10), and 500/510 Whispers make sense. If you're only shooting at short range a pistol cartridge wouild be easier, perhaps 40 S&W 45 ACP, or even 50 Beowulf.

All around I see little if any pros for the 6.8 SPC as a dedicated subsonic compared to a 300-221. You can easily make 300-221 brass from 17 rem, 221 or 223 cases. Already prepared 300 Whisper cases are pretty easy to find too. Have you looked on Gunbroker.com?