I added extended magazine to my weathered 10 mm pistol that came as a download bonus.
Now when I equip it my screen goes all white and the weapon is invisible in my hand.
It will still fire but I can not see it.
I went into third person and
I have a large red square around me with a large white ( ! ) exclamation mark in it.
What is this all about?
Anyone else had this issue?

Well known bug, either the meshes are missing or it was never intended to be modded, reload a previous save.

The same thing happened to me on my first play through, only I added the laser sight to the gun. Maybe it really isn't meant to be modded, but it kind of happens anyway and is completely wrong. I don't think that any special weapons can be modified. I tried adding a scope to that special laser rifle, but I couldn't do it.

I have modded the all American and no issues there, it's just the weathered 10mm that i know of

You can't mod the All American.[or the regular marksman carbine for that matter].

could have swore it was the all American but it has been a while since I played so ill have to check what one I was thinkin of