Modification of clubs is a waste of time and can result in your self-defense claim going out the window and you ending up being charged with a homicide. Establishing intent is much easier when your club looks like something from Hollywood.
This is completely wrong. This is why:
Find a court case where the weapon a person used was used to prove they were guilty. Over 2 years on this site, and hundreds of hours of research in law and I have never run across such a thing.
A bat will be considered deadly force if its within its delivery system range and used in the manor that could cause death or great bodily harm. If someone is 10 feet away and has a bat (at the ready) and is advancing towards you (with the obvious intent) then you have a case of someone trying to use deadly force on you.
Almost anything can be used to kill another person. Some things are more lethal then others in the deadly force category (or should I say the chance of survival varies). Adding nails to a bat increases its chances of causing death or great bodily harm. Mainly because the nails could easily puncture the heart/lungs/arteries/etc. However making it a more effective weapon does not change the fact that it is considered deadly force when used with intent.
The weapon you use, be it a bat with nails, a plain bat, or a truck, can't be used as evidence that a person is guilty of murder. If you were justified in swinging a bat at someone's head, then you would be just as justified with swinging a bat with 20 spikes in it. The law doesn't state that there are multiple levels of deadly force, and your only allowed to use certain levels against certain threats (such as level 1 for bats, level 2 for bats with nails, level 3 for guns, level 4 for vehicles, level 5 for bombs, etc). If you are faced with a deadly force threat, you can counter that with deadly force of your own, be it a bat with nails or a plain bat.
There is one possible potential problem with putting nails in a bat. Much like how an baton an officer uses is not considered deadly force when used as trained (the training requires the baton to be aimed below the waist in most cases). If a person were to take a blow to the head and die from a baton (say if the person were to lunge during the swing, which resulted in them being hit in the head) the officer will likely be in the clear because the intent wasn't there. In the case of a bat, the question of the bat being capable of causing death or great bodily harm is easier to answer with nails in it, even if the bat is small. Even so a standard bat could easily cause great bodily harm or death, and its far more capable of doing such things then a baton.
Any remote/knife edge/cliff hanging/1 in a million chance you have of convincing a jury that you swinging a standard louisville slugger was in a manner that would normally not cause death or great bodily harm (Such as in a case where you claim you were using the bat with the intent to keep a person back, and not in a way that that would cause death or great bodily harm) would likely be eliminated with nails since the chance for death or great bodily harm is much higher with nails. Again thats going to be hard because your average person swinging a bat full tilt at a average person could easily cause great bodily harm at a minimum even with swings to places other then the face.
The reality is you shouldn't use a bat to combat anything other then a deadly force threat. You also shouldn't put nails in your bat because it really wont help you a much. A mace is even worse since with minimum training you would likely be hard pressed not to tenderize yourself.