Go to _damage.gsc. Find 1053 line.
[php]<...>
Callback_PlayerDamage_internal( eInflictor, eAttacker, victim, iDamage, iDFlags, sMeansOfDeath, sWeapon, vPoint, vDir, sHitLoc, psOffsetTime )
{
if((sMeansOfDeath == "MOD_MELEE") && !(isSubStr(sWeapon, "riotshield"))) {
iDamage = 700;
}
<...>[/php]
P.S.: Use search.