I have a follower coming from ebay also to modify one of my existing 223 30 round metal mags. I've read reports of guys saying they hold as little as 9 rounds and as high as 12 rounds without malfunction. If I can get 10 to feed consistently, I'll be pretty happy.
I don't know how often I'll need that many rounds of 450BM in a single mag, but it'll be nice to have that option. Especially considering I have a tube fed 12 gauge that holds 10 rounds; sure seems like we should be able to have at least 10 rounds for any cartridge shot from an AR platform!