I've now looked at it. They are idiots. [Some even think it was 'fired' by the Confederacy, yet the round was completely intact.]
We know that Hazlett's 5th United States Artillery, Battery D was the only Union artillery there. We know that the battery was equipped with Parrott 10 pounders. The three inch version of the Parrott was not made until 1864. Therefore, it must be the 2.9" Dyer (
http://www.civilwarartillery.com/projectiles/rifled/IIIA60.htm) and not the 3" Dyer (
http://www.civilwarartillery.com/projectiles/rifled/FAOIIIb10.htm).
[Annoyingly, neither of them exactly correspond in measurement to their names; 2.9" Dyer is 2.93" and the 3" Dyer is 2.96". I'm a member of CivilWarTalk and I'm happy to argue many issues that are raised there. However, although it's tempting, I'm not going to argue with them over fractions of an inch. The history should tell them what it was given who was there in 1863 and when various ordinance came into existence. It would have been preferable for the NPS to properly examine and record details before rushing to blow it up (bdtex got it right about that on page 2, Post #33). It had been there for almost 160 years so it's unlikely that it would suddenly explode in the short time it would have taken to examine it (and they obviously took time to pick it up and position it for photographs).]