[CareFully]

PHP Code:

#include <sourcemod>
#include <sdkhooks> 
#include <sdktools>

new g_PVMid[MAXPLAYERS]; // Predicted ViewModel ID's
new g_iSMGModel;    // Custom ViewModel index

public OnPluginStart()
{
    HookEvent("player_spawn", Event_PlayerSpawn);
}

public OnConfigsExecuted()
{
    g_iSMGModel = PrecacheModel("models/weapons/paintball/v_pbgunt.mdl"); // Custom model
}

public OnClientPostAdminCheck(client){
    SDKHook(client, SDKHook_WeaponSwitchPost, OnClientWeaponSwitchPost);    
}

public OnClientWeaponSwitchPost(client, wpnid)
{
    
    decl String:szWpn[64];
    GetEntityClassname(wpnid,szWpn,sizeof(szWpn));
    
    if(StrEqual(szWpn, "weapon_mac10")){
        SetEntProp(wpnid, Prop_Send, "m_nModelIndex", 0);
        SetEntProp(g_PVMid[client], Prop_Send, "m_nModelIndex", g_iSMGModel);
    }
}

public Event_PlayerSpawn(Handle:event, const String:name[], bool:dontBroadcast)
{
    new client = GetClientOfUserId(GetEventInt(event, "userid"));
    g_PVMid[client] = Weapon_GetViewModelIndex(client, -1);
    
} 

// Thanks to gubka for these 2 functions below.

// Get model index and prevent server from crash
Weapon_GetViewModelIndex(client, sIndex)
{
    while ((sIndex = FindEntityByClassname2(sIndex, "predicted_viewmodel")) != -1)
    {
        new Owner = GetEntPropEnt(sIndex, Prop_Send, "m_hOwner");
        
        if (Owner != client)
            continue;
        
        return sIndex;
    }
    return -1;
}
// Get entity name
FindEntityByClassname2(sStartEnt, String:szClassname[])
{
    while (sStartEnt > -1 && !IsValidEntity(sStartEnt)) sStartEnt--;
    return FindEntityByClassname(sStartEnt, szClassname);
} 


I've tinkered around with cs:go trying to get custom viewmodels working without flickering or turning off prediction and after hours and hours I think i finally got something that seems to work.

It might work with other mods aswell, but i haven't tested it.

The custom model must have animations in the same order as the original model, oherwise it's going to play wrong animations.

EDIT: I added a custom model to attachments to test the code. It's a paintball gun converted from cs 1.6 to cs:go. Lighting of the model might be a bit glicthy .