Do you have a black jack or a sap? A blackjack has a round head and braided leather on the handle. A sap is a sucker shaped piece of lead bonded to a steel spring with leather sewn over it.
Until the 70's most police uniforms had a "jack" pocket sewn below and just to the front of the right rear pocket. Either weapon is really brutal when the face or head is struck. My first "moving" assignment was driving a paddy wagon. I have seen some severely lacerated prisoners who suffered a couple of blows from a jack. If you choose to use a sap or jack, check your local laws. As far as I can tell, they are outlawed in most jurisdictions. If you can carry one of these, think of it as a short stick. Jabs work very well in tight situations. Any overhand blows should be delivered to a large muscle or joint. Shins are also a good target.
A truly impressive weapon is a sap glove. Most victims of a sap glove think you are as strong a king kong to land such a forceful blow and tend to calm down after a couple of blows. I remember a 125 lb female officer decking a three hundred pound street punk with two blows. She got a lot of respect on her beat after that.
Most agencies have got away from using these type of weapons. Police are now better trained in verbal skills, locks and joint manipulations. The expandable baton has replaced most all sticks and jacks now.
Always remember, if you pull one of these out of your pocket, someone will be justified to pull out a gun. Also, in the eyes of our culture(cops, witnesses, magistrates, judges, media, and jury members), your use of this type of weapon automatically makes you suspect. Gentlemen use pistols, thugs use knives and clubs.