There is no limit. To suggest that there is a limit implies that the right to life and what necessarily extends from it -the right to self defense; is limited.
Beyond that -who sets such limits IF they do exist? The right to life is endowed all by God -where does government fit in to this right? Per the Constitution, government does not grant the right -it only guarantees it and defends it. In fact, it is primarily the right to life that is the very premise underlying the arming of government, military and police forces. What limit does government have in protecting this right?
Remember, it was not the Federal Government or the States that constituted the government -it was the people; the same people that revolted against the king. The three branches of the Republic are the Federal, the States, and the People -all bearing arms for the same purpose.
So you mean a civilian can shoot and kill a person with a howitzer, or even a tank?