If you're learning martial arts for self-defense in 21'st century you're wasting your time. Your karate skills won't help vs someone with a led pipe, crowbar or a knife.
Crow bar is a long range weapon, so get in close. Knife is probably the most difficult to defend against - certainly more so than a gun.
I did this exercise once, where my training partner held a paintball gun a couple inches from my chest. My job was to disarm him without getting shot. His job was to shoot as soon as he saw me move. It took five attempts before I got it. And getting hit at that range is a great incentive - all we had on was a facemask and overalls over shorts and a t-shirt.