IIRC grenades and the like are destructive devices and illegal.
If one is to “prep”, even if ones “prep” is for a civil war, there is little need to build these illegal things now.
Hoarding common ammunition is legal, much more practical and to the point.
Because the concept and knack on how to do so will magically appear in your brain?
And it is much better to do it later when there are no emergency rooms around to help in case of an "oops!"
Which raises an interesting point. The Founders gave us the 2nd Amendment in order to protect/defend ourselves against a totalitarian and/or oppressive government. If the citizenry is to be able to protect/defend itself against such a government, should not the citizens have the same weapons as government?