All we think that we know (i.e., unconfirmed by official sources) is that the holes in the tree were the size of 16-penny nail heads, not necessarily the size of the nails themselves. Terrorists in Israel have been known to cut off the straight part of the nail and just use the nail head with a short, sharp stub.
Less mass + smaller profile = deeper penetration of flesh
Check out this photo of the blast site from Northeast Intelligence Network. I want to say that I see pock marks on the near side of the concrete -- Hinrichs was apparently sitting on the left side of the park bench -- but I can't tell for sure. Enlarging the picture doesn't help either or I would have posted an enlarged version.
Stingray blogsite: Conservative Christian Commentary
