My husband takes a .223 Thompson Contender with him when he hunts turkey. It's a relatively long pistol, but he can easily conceal it in a holster under a jacket. The gun has a scope, probably sighted in at 200 yards, that places a red dot on the target (btw, no scopes allowed turkey hunting ). An weapon of this time would be easy to conceal and the killer, as you speculate, could simply walk away.
What are its dimensions?