You'll need to buy every incremental upgrade for a weapon first, then buy this final upgrade with either more money (typically another 60-80k on top of the other upgrades, but it varies per weapon) or an upgrade ticket (trade 30 spinels for the first one, 40 for a second one).
Or if you buy the DLC upgrade tickets you can apparently shortcut to the full upgrade without spending any (in-game) money or spinels.