The main conflict in the story occurs within the main character, Dave, as he struggles to prove that he is an adult worth respecting and admiring.  That conflict is definitely internal, and is the underlying reason behind all of the external conflicts that occur.  It lies at the root of the plot.

For example, the story starts off with Dave in the fields, feeling dejected and disrespected.  He doesn't like how the other sharecroppers treat him like a little kid and tease him; he wants to be treated like an adult, like a man.  This bugs him so much that he spends a great part of his day fantasizing about earning their respect; he feels he can do this if he has a gun.  So he goes about manipulating and strategizing about how to do this.  This creates external conflict--lying to his mother, hiding the gun from his father, and then his excursion into the fields with the mule, Jenny, and the gun.  He has to be able to shoot the gun, which he feels will earn him respect, and his "practicing" ends up in the mule dying, and the resulting conflict with his boss and family.  All of this stemmed from the fact that Dave had an internal issue of insecurity, and wanting to be a man in the eyes of others.  He even holds this pride and victim attitude at the end as he flees his responsibilities rather than be looked down upon by his family and other workers.

I hope that helped a bit; good luck!