I bet he had no idea. In fact, I know he didn't...that's why he designed the AR-18 as an improvement on the 15. And the Stoner 63 as well, of course. You can get pretty close to that with the Robinson M96 Expeditionary Rifle, and the interchangable barrels and easily replacable feed block makes an adaptation of a more reliable magazine in one of the more effective cartrides an easy change from the 5,56mm standard.
I've heard that the Stoner 63 really wasn't all it was cracked up to be. The Robinson, on the other hand, is supposedly excellent.
I'd have bought one by now, if the price was a little less.
Sigh. One income (E-6 pay) and a new baby have pretty much negated the weapons purchases for now.