Hi,
___________________WARNING ALERT____
Since more then one dude is asking how to make this, it is fairly easy, but requires editing the files, so there is 2 possible solutions after you edit files:

1) You edit the steamid number of the mod thus giving yourself a new mod which will not be upgraded if developer of the mod will issue an upgrade to steam

2) Your edited .ini files will be upgraded to the developers upgrade on the next update of the mod, so you will loose your work and will have to redo it everytime you want certain changes to be made.
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Here is how to make your assault rifle upgrades tied to another weapon.

1)You go to your steam/steamapps/workshop/content/268500/ and here you can write your mods name in the search field it should give you the ability to find the mod.

2) You open the said mod, then navigate to src/*modname*/classes , and you should see X2Item*itemname*

3) Open it with notepad and you will see one of the config files for the weapon, press ctrl+f and search for " Template.UpgradeItem " this will show what template Tier 1 uses to upgrade to Tier2 weapon, for making this tied to Vanilla upgrades it should say " Template.UpgradeItem = 'AssaultRifle_MG'; "

4) Next , search for " Template.CreatorTemplateName " , this says what schematics will the gun use to upgrade from Tier 1 to Tier 2 , for making this tied to Vanilla upgrades it should say "Template.CreatorTemplateName = 'AssaultRifle_MG_Schematic';

5) Next u need to search for "Template.BaseItem " , it shows what previous item is needed for the upgrade to take place, for making this tied to Vanilla upgrades it should say " Template.BaseItem = 'AssaultRifle_CV'; "

6) Repeat 3-6 for T2 -> T3, just instead of Magnetic you put in Beam and base item should be magnetic

7) Save the ini file, this weapon is now tied to vanilla upgrades.