So, I've now successfully modified a Magpul PMag to accept a total of 29 rounds and feed as reliably as it does in 5.56NATO. I think it would work 30 rounds just fine, but since the rim side of the 5.45x39mm is bigger than 5.56x45mm, it just won't allow any more room for another round. I had to modify the precious PMag follower to successfully convert the magazine to 5.45x39mm. And from what I can see, it seems to only work correctly in the windowed magazines. The standard PMags don't seem to fit in as well. I'll add the specifics when I get the pics up.