Some of Mann's testimony in the 1880's, before the "Logan Board."
"Now, to go back. In 18ß2 I brought to the city of Washington a 3- iuch breech-loading steel field-piece, that was fired here at Battery Fox under the direction of Admiral Dahlgren 437 times. There is a representation of that field piece [exhibiting photographs of the gun]. During the firing, when the men who had charge of the gun were down in the hollow, the officer in charge and myself sitting in a tent on the hill near by in conversation—it was a hot July afternoon—and the gun was flred 96 rounds in 70 minutes without interruption ; that is, without any interruption of the working of the gun whatever ; and the men who fired the gun afterwards said if they had known we were noting the time they could have fired it three times as fast—96 rounds in 76 minutes. Upon this successful trial of that field gun to that extent I accepted an order from Admiral Dahlgren for an 8-inch breech-loader rifle, to be made on the same plan. Mind you, this was in 1862, when there was no 8-inch rifle of any kind in this country that was known to be reliable. I took a contract to make that gun at my own risk and responsibility. The gun was made and completed in September, 1863, and here are photographs [exhibiting photographs of the 8-inch gun] of the gun as it was fired on the proving-ground, an 8-iuch gun made in 1863 and tested."
The full report is here:
http://books.google.com/books?id=rXYDAAAAYAAJ&pg=PP7#v=onepage&q=&f=falseI'd like to find the images he presented showing the gun tested under supervision of Adm. Dahlgren in 1862. Mann said it was a 3" bore, I wonder if 21 years or so could have clouded his memory and it was really one two I'm studying?
Skimming quickly over Mann's testimony a lot of it appears to be "dirty laundry" within the ordnance department, which he was airing in front of a Congressional committee to enhance his own stature, or discredit the department. It looks like he had a particular desire to discredit Col. Crispin.
There are lots of interesting drawings in the book linked above, mostly after about page 130. The drawings before that are mostly fold-out plates, and as you may know, Google doesn't take time to fold them out so you can only see the edge of the page and usually only 10% of the image if that much.