I've been trying to work up some 300 BLK loads and have not had success yet (haven't been able to get factory ammo accuracy). Because they were cheap at the Gander goob sale, I bought some Hornaday SP Interlock. My initial loads with them had bad accuracy, but so has most of my other stuff. Reading the article below, I'm wonder if they're appropriate for autoloaders?
http://www.massreloading.com/twobullets.html
http://www.massreloading.com/twobullets.html