From Halopedia, the Halo wiki
While the tactical training rounds typically used in the training of normal infantry are sometimes also called stun rounds, the stun ammunition used in the SPARTAN-IIs' and IIIs' training are a distinct type of round, being rubber bullets which numb the targets they hit by the sheer force of the impact alone rather than an anesthetic in the case of the TTR round. As such, these rounds are potentially lethal at point-blank ranges. Other nonlethal ammunition, including paint pellet rounds or narq-darts, were also used in the SPARTAN-IIs' training. Meanwhile, the rounds used in the training of the SPARTAN-IVs seemingly inflicted a powerful electric shock on their targets upon impact.