No, You're the one who stated what you think is a fact, so prove your assertions with historical evidence. Show us where, anywhere, where the army stated on paper, or in a recorded statement, that the horse killing abilities of the cartridge were a primary condition to the adoption of the cartridge.
And don't give me more magazine articles written by gun writers nobody ever heard of, quoting bogus ballistics. Give me actual forensic Army documents stating that the cartridge had to be a horse killer.
I think this whole notion is a myth, repeated by generation after generation, until it has been repeated so many times, and read by so many people, that everybody accepts it as truth.
And FWIW, I hate horses. My sister is dead because of one.