1000 yards? Really?

What you're talking about here (a .308 with a .224 bullet in a sabot) isn't much different than a .22-250, just a shade hotter.

Just a quick check in LoadBase tells me that a 50 gr (BC = .232) .224 V-max driven at 4000 fps (which is about what I expect to achieve in velocity) will go subsonic at 760 yards, and drops a mere 421.1 inches (35 feet) at 1000 yards. Maybe I need to get a tapered base to get that much elevation out of my optics?

I've used a .22-250 out to 600 yards on the KD, but what steam it had left was barely enough to take the paint of the 12x12" steel plate.

The "die" you're probably referring to is the one used to seat the .224 bullets into the sabot. I've not tried it yet, but it looks to be a much better alternative to stuffing bullets into plastic sabots by hand.

There's actually some information out on the internet regarding such loads. I've had plenty of projects ahead of this one, not to mention plenty of loaded .223 and .22-250 to go through at the local dog towns before it's likely I'll load up any .308 sabots. It'd be interesting to load some up in my .300 Weatherby as well, but I'm guessing it'll be far from practical.