If all things are perfect the 243 is sufficient with proper bullet choice. Such as, a 100 grain controlled expansion bullet. Perfect would mean broadside with no obstructions in the way and no wind to drift the bullet. Also, using a range finder to know the exact distance to the target. Then you must be able to execute the shot without making a mistake. Mule deer can and have been taken with the 243. I would opt for something larger if I have the opportunity.