Originally Posted by Armageddon

Each gun's iron-sight is coded for a specific model. If you replace the model of the gun and that model does not have the same origin and dimensions among other things, the iron-sight will not work correctly.
I know of no way to fix this other than editing the model or the LUA code for the iron-sights. But, since the LUA code is server sided, there's not a way for you to modify it for your client. Not a legitimate way anyways.

WHAT EVER THE FUCK YOU JUST SAID!