Fall 2014 update:
A while back Green Mountain had their 20" .500" blanks on sale, plus free shipping. I snagged one to use instead of the 15" barrel I bought back, uh, the other day. That one would have needed a brake to meet minimum length.
I picked up an RPK flat from AK-Builder; it's identical to the Yugo flat, except it has the AK-74 style magwell. I picked up some AK-74 mags. I also got some .jacketed .50 caliber bullets to load some dummy rounds with, to decide which way I want to go for magazines.
Tujungatoes mentioned some feed problems with the .223 AK mags in his thread over on calguns; I need to ping him and see if he got that sorted out.
My loading press and dies are over at a friend's house; I need to get over there to load the dummies. I'm going to bring my calipers along to measure the barrel of his Saiga-12. I have two Beowulf builds backed up, plus he's piggybacking two more, so we're going to need several gas blocks. We might use the Saiga gas block diameter if it's compatible with the .50 profile.
I found the Alexander Arms .50 Beowulf AR barrel profile; I'll stick close to that if possible.
In further news, I'm reorganizing things to I'll have as much as, oh, about two hours per week to devote to gun toys. Which is way more than the "none" I've been able to get for the last few years...