From the description given of Custer's body, there's always the possibility of instantaneous rigor mortis.

The current thinking by pathologists seems to be that there is no such thing, but there are several very interesting descriptions of cases during the Civil War in which people instantly froze in the position in which they were in at the moment of death. Lt. Charles Gentsch, M. D., had a very interesting paper on the subject published in the Ohio MOLLUS papers (see https://books.google.com/books?id=8g8CAAAAYAAJ&pg=PA146&lpg=PA146&dq=instantaneous+rigor-mortis+gentsch&source=bl&ots=hSDHF2TN3i&sig=B37XTEA7iyL9eowJOxU3jt811aM&hl=en&sa=X&sqi=2&ved=0ahUKEwizluyS6NDPAhUB4CYKHe1wCeQQ6AEIJDAC#v=onepage&q=instantaneous%20rigor-mortis%20gentsch&f=false ) The link is not to the complete paper, but will give you a sense of what it was about. The complete paper is fascinating.