The rarity scale should be updated on a number of projectiles, particularly the 5.1 and 6.0 Hotchkiss shells. Having decided to fill out my Navy projectile collection, I have had no luck on obtaining a 5.1 or 6.0-inch Hotchkiss. Based on contacts I have made in the last 6 months, I would guess that the 5.1 is a rarity 9 (I know the collections where more than 10 are) and the 6.0 a rarity of 10 (I only know of less than a half dozen).
As for other projectiles such as the 15-inch Dahlgren shells, a large number have been recovered since the book was published. The triple-fuzed 15s may still be a 10, but my guess it is a 9. However, the the cored shot (I only know of one - at the Citadel), the Dahlgren solid shot (of which none have been reported recovered - these are the ones without the "ears;" the Rodman solid shot are the ones with the "ears"), and the 15-inch Dahlgren case shot (which has the Bormann fuze) are definitely 10s.) More of the 7-inch James shells have been recovered in the last 13 years since the book was published, but only a few of the 5.82s and 6.4s I would guess that the 7s are a 7 and the 5.82 is an 8 and the 6.4 is a 9.
When I was preparing the book, I consulted with a large number of dealers and collectors on rarity gradings, and some opinions varied widely. We learn more every year, particularly when a number of new recoveries are reported. This particularly happened with the 15-inch Dahlgren shells, at least a dozen of which were recovered from Charleston Harbor after 2003. On the 5.1 and 6.0 Hotchkiss shells, I may have been influenced by the finds at Roanoke Island, which have never filtered into the market, but do exist and have survived.
New type finds are always exciting. And there have been a fair number. I personally found an 8-inch Parrott case shot with the large Parrott fuze at the Mansfield Show several years ago. And 5.82 and 7-inch tear-drop Read bolts have turned up. A 4.5 Dyer bolt showed up at the Mansfield Show the weekend my book debuted, and of course, it wasn't in the book except in the Missing In Action section.
Large holdings of some types of projectiles by either government museums or by some private collections, affect the rarity ratings , even though the projectiles may never come on the market.
Hopes this answers some of the questions you have on rarity ratings
