You can set the line end and join styles for objects that have lineweight assigned as a plot style override. 
-  Click 
	   .  Find .  Find
- Right-click a CTB or STB file. 
  
- Select Open. 
  
- In the Plot Style Table Editor, Form View tab, click the Line End Style or Line Join Style field for the plot style you want to change. 
  
- Select an option from the drop-down list. 
  
Note: The "Use object end style" option displays rounded ends. It cannot be set as an object property as the name might suggest.