Hi there. All weapons in the game have a maximum ammo reserve and clip capacity that can't be surpassed with mods. The maximums are generally by weapon archetype, with exotics being an exception since they can be tuned individually. For example, Xenophage fires individual rounds instead of a spray of bullets so it has different capacities than legendary machine guns. Its max reserve capacity is 32 bullets. Weapons with small reserves or have near max reserves to start with will receive lesser or no benefit at all from stacking mods based on the math, such as 1k Voices. Mods that do stack have diminishing returns. For example, a second scavenger mod will only increase the effect by 50%, not double it. Bungie hasn't provided a curated list of which mods stack and which don't, except for those that specifically state they do not stack in their descriptions. Bungie occasionally puts out patches that change ammo capacities and/or mod effects, which is why there isn't a complete list, but patch notes always explain which weapons/mods are affected and how. Additionally, some mods are affected differently in PvP based on how the ammo economy works. For example, special ammo bricks will always drop from opponents killed regardless of what mods you equip to increase chances and no mods can spawn heavy bricks from kills on their own. Secondly, the amount of ammo you can receive per brick is specific to the type of weapon you have equipped and that cannot be altered by Scavenger or Reserve mods.

Hi there. What VictorH said is correct—weapons have a hidden stat that determines their maximum ammo capacity, and sometimes these weapons are already at their maximum or they reach it within one reserves mod. I was trying to find the source for this, but unfortunately couldn't. This is typically seen with exotic weapons that have a limited ammo capacity such as The Wardcliff Coil and One Thousand Voices.