1) Open MW3
2) Go to Barracks*
3) Open the editor
4) Choose your preferred attachments
5) Click Set, close the program and go pwn!
Note: If weapon doesn't support the attachment, then it will disappear in-game. Only use attachments that are valid for the weapon, example: assault rifles can't have variable zoom or grip.
*If you don't do this in barracks, you have a chance of getting vac banned.
This tool is for version 1.4.382 only, if you want to be really safe, you should check this thread:
https://www.mpgh.net/forum/404-call-d...rsion-mw3.html and see if update has occured.
Source code included in the attachment, feel free to redistribute or modify it anyway you want to.