] Yes, I imagined pistols. And I cannot imagine that the Colonel, being a soldier, would miss what he aimed for. So my speculation is that Willoughby "deloped", ie shot into the air which was an admission that he was at fault, that he accepted that he did wrong. Brandon, being the honorable man he is, would have afterwards done the same, for he could not then shoot an unarmed man, just standing there.
"You're not worth the powder!" a la Horatio Hornblower. (Has anyone seen that? Jolly good show!)
Seriously, though - the Colonel is a soldier and a spotsman; I doubt very much that he missed unintentionally. He must have wrestled with the thought of killing his loves beloved scoundrel all the way up to the spot.