void AddSilencer( void );

This defines a new CGlock member function AddSilencer. Then replace the CGlock::SecondaryAttack function (About line 110) with this one:

void CGlock::SecondaryAttack( void )
{
    m_flTimeWeaponIdle = gpGlobals->time + 2.0f;
    m_flNextPrimaryAttack = m_flNextSecondaryAttack = gpGlobals->time + 2.0f;
    SetThink( AddSilencer );
    pev->nextthink = gpGlobals->time + 1.0f;
    SendWeaponAnim( GLOCK_HOLSTER );
}

This sends the holster animation to the weapon model (Which causes the weapon to drop out of sight), sets the think function to AddSilencer (The member function we just added), and sets the next think time to the current time plus 1 second. There're a few variables and functions you might not recognize in the new Secondary attack function, so they're explained below.

void *m_pfnThink, float nextthink, and the SetThink macro: for those of you who've never coded Quake 1 or 2 mods before, the think function is a void function pointer. You can set it to any member function from the current class that doesn't take any parameters or return a value. You then set the nextthink value to the time when the think function should be executed.

float gpGlobals->time: The current time in seconds. This is a floating point member of gpGlobals.

float m_flNextPrimaryAttack: This should be set to a time value, much like next think. If gpGlobals->time is less than m_flNextPrimaryAttack primary fire commands will be ignored, this prevents the gun from firing as you fit the silencer on.

float m_flTimeWeaponIdle: If this value is less than gpGlobals->time (If it's "already happened") then the weapon idle animation takes over, cancelling any currently running animations. It's important to set this to the right value, or the holstering and drawing animations will be interrupted.

virtual void SendWeaponAnim( int iAnim ): This function starts a weapon animation. All the weapon animations are declared in the enum at the top of their respective cpp files.

entvars_t *pev: This class member of CBaseEntity (The class pretty much every animated thing is derived from) stores important entity data.

Okay, now add this just under the function we've just declared:

void CGlock::AddSilencer( void )
{
    if(pev->body == 1)
        pev->body = 0;
    else
        pev->body = 1;
    SendWeaponAnim( GLOCK_DRAW );
}

some models have multiple meshes, pev->body is the current mesh of the weapon. Changes made to pev->body won't show up until the next animation is run. For the glock, body 0 is the glock without the silencer, and body 1 is the glock with the silencer.