That is one damn interesting letter! They had the idea to create a submarine at that time?!
The final part about the 8 bullet gun was also interesting also. Sounds like it would just fire every few seconds until all the bullets were out. But the project was scrapped as the inventor was asking too much to build them. I'm kind of surprised they had that idea, but didn't make an 'official' one until much later. Maybe it came down to the definition of what a semi-auto rifle is.
Very cool find!