Not sure exactly what you are referring to by "flat ends," do you mean how the lines plot out? You can control this in the plot style settings by changing the "line end style" settings.
Making sure the .shp file lines plot out with the butt ends is important. However, I am in fact looking for a way to make the .shp file lines look as if they are basic AutoCAD polylines with the flat butted ends in the drawing area. I am using linewidths for my .shp lines and they appear with round ends. Thank you for your reply, I will utilize the Plot Style Table Editor in the future.