Weapon mods after bronze

I'm not 100% sure what you're asking, so I'll answer broadly.

There are two different categories of weapon mods: those that drop in packs and those which can only be purchased with Mission Funds. The mobile app, for context, references them as "mods" and "mod variants," respectively. I'm not entirely sure if the former will drop from MF purchases, but the latter definitely do not drop in card packs. Mods have bonuses, while mod variants (with the lone exception of the pistol scopeless accuracy mod) all have penalties (e.g. to weight or stability) to offset their bonuses.

I don't know the specific drop mechanics/priority with certainty anymore, since I had my mods maxed pre-1.09, but these are my recollections based on what others have said.

The uncommon mods are very useful, but are also difficult to max, particularly after 1.09. They don't drop until after all of your uncommon characters and weapons (and, I presume, their variants) have been maxed. They include:

AR Magazine
AR Receiver
Pistol Magazine
Pistol Receiver
Shotgun Receiver
Shotgun Spare Ammo
Sniper Receiver
Sniper Spare Ammo

Rares will drop more readily (you don't have go have maxed all rare characters/weapons/variants, IIRC), but are more expensive per unit due to the packs from which they drop. They include:

AR Barrel
Pistol Barrel
Shotgun Barrel
Sniper Barrel

We should hire the biobots to fix the penetration problem with pistol weapon mods.

We'd never have any idea what they were doing, and it wouldn't make any sense. But... hear me out... our expectations would more or less be met!