I bought the 12.7mm smg and then bought the 12.7 smg laser scope. the scope doesn't come up when i hover over the gun and type x. Ive never modded a gun so i dont know if im doing something wrong. the scope is in my mics items so its def there.

You must have Gun Runner's Arsenal installed.

They implemented it very poorly. THere are a handful of instances where they added in mods for preexisting weapons, but won't work with normal weapons. The mod should have (GRA) next to it. That means it can ONLY be added to a 12.7 mm SMG (GRA). The normal variant won't accept the mod, as you found out.

This really svcks when you have a mod that removes the "GRA" from the lists....

BTW, there are also mods out there that remove this issue with modding the guns.