Mari:
I thought the cowboys would be on their feet anyways, resting their rifles on something at least in the beginning?
Why? It's the (relatively) open terrain* during the day. Unless the Comanches can sneak up to 80 yards before being noticed there is no reason to be sitting around.
* Presumed. If it's different terrain or circumstances, that changes things. And with 80 yards distance everyone could easily be mounted and riding by the time the Comanches get to 20 yards, unless the Cowboys are being denied horses for some reason.