How to increase weapon mod capacity?

[JerlBays]

Some weapons have 60 mod slots and those weapons are super powerful, however other weapons only have 30. How do you increase the mod slots?

[Watermelons]

All weapons can get 60 mod slots, you have to put an Orokin Catalyst on them. Otherwise they have up to 30 standard, at level 30. You can also get the most out of your slots with Forma to add more polarity slots, cutting down on the space mods use.

If you purchase any weapons with platinum through the market, or warframes/sentinels for that matter, they usually come preinstalled with a catalyst/reactor

[SolarSabotage]

Go to "Actions" in the bottom right corner of the upgrade menu. At the left it says "Install Orokin Catalyst". If you have a catalyst, installing it will double the mod capacity, making the max slot capacity 60 instead of 30.