That's a great question, the bipoly's creator determined that including a hole for a buffer detent would lower strength so there is none, you hold it back while you install the upper.
Like I said, it makes no sacrifices on strength. My pla version is well over a year old with thousands of rounds through it and I'm not the only person with these results. I think he was on a good path with this one.

You're gonna love it. I'm gearing up to print it in one piece, no seam like a Monopoly.