g_weapon.c

The following lines in the weapon_grenadelauncher_fire function adds alittle height when you fire off a grenade, so comment them out by adding a // infront of them, that will make them fire just straight forward where you point to.

// extra vertical velocity
// forward[2] += 0.2;
// VectorNormalize( forward );

Lets make the grenades fly straight and not effected by gravity and fall to the floor, so in the fire_grenade find the line:-
bolt->s.pos.trType = TR_GRAVITY;

and change it to
bolt->s.pos.trType = TR_LINEAR;