I had posted a question about the Scavi tour before we went, but received no replies, so I thought I would share our experience here.
Getting tickets is pretty well documented elsewhere, and here is the official site: http://www.scavi.va/content/scavi/en/ufficio-scavi.html
Our questions were about how it really worked, compared to what the Vatican site says. Here’s how it went for us in October 2019:
We arrived at about 8:30 for a 9:15 tour, and the line to enter St. Peters for general access was already more than half way around the obelisk. No way we would get thru that line in time. We were pretty disappointed, as we thought we had to check our small backpack and cameras BEFORE entering the Scavi. That’s not how it turned out.
The rules seemed more relaxed than stated on the website. The Scavi has its own security line, which is basically empty as there are so few guests taking the tour. If you look at St. Peter’s square on a map, north up, you get to the Scavi security at about the 8 o’clock position on the colonnade (you may need to wiggle around the metal fence). We found it, still worried about getting the cameras / backpack thru. We were still about 30 minutes early, and were told to get into the security line (not really a line, as it’s basically empty) no more than 10 minutes before the tour. So we waited, watching people go thru for the 8:45 and 9:00 tours, and saw plenty of small backpacks and handbags go thru.
Turns out that we had no problems at all. My small slingpack holding a small point and shoot and 2 iPhones was no problem. I’m sure that there is a size limit, but we had no problem. No photos allowed, so leave the cameras packed away. There were only 11 on our tour, and some had slightly bigger packs then us. Still no problem. Everything goes thru an X-Ray, so they knew what we had.
The tour was great. Tight passages, humid, and kind of dark, but great. And you do get to go thru most of the Papal Grottos at the end, and you end up in St. Peters without standing in any line. So, even if you have minimal interest in the tour, for about $15 you can skip the entire hours-long general entrance line. Best deal ever!