Welp fallout 4 is epic, but it is sad that you can custimize every aspect of your weapon except for ammo and looks. As ammo mods have existed before I really want them, and I am sure I am not alone.
example imagine taking an automatic combat shotgun and loading its drum with one of the following.
shells
standard ammo
(here for list complete would be craftible like ammo)
slugs
increased range and accuracy, lower damage.
gun nut 1.
rifled slugs
superior range and accuracy.
gun nut 2
dragons breath
shots a cone of fire instead pellets.
gun nut 2, explosives 1
explosive slugs
increased range and accuracy, superior damage
note: explodes on impact
gun nut 2, explosives 2
explosive rifled slugs
superior range and accuracy, superior damage
gun nut 4, explosives 4
these ammo mods would have a bonus to damage for teh weapon, but unlike resiver bonuses you can swap them out, for example explosive rifled slugs would be expensive to craft and would be hard to come by, as such you may want to use standard slugs or basic buckshot for most situations. the rule of thumb is the better the ammo the harder it is to come by.
another example 10mm rounds
10mm
standard
subsonic 10mm
lower damage and range, harder to detect in stealth
note: combined with suppressor makes it almost impossible to detect where the shot came from.
gun nut 1
high powder 10mm
superior damage, lower range and accuracy
gun nut 1
incendiary 10mm
lower damage, chance to light enemies on fire.
gun nut 1, explosives 1
10mm clean
pre-war grade 10mm ammo hard to come by even harder to make. (superior damage)
gun nut 2
ratshot 10mm
lower damage and range.
note: causes projectile to shoot 8 pellets which in stealth do heavy damage out of stealth useless.
gun nut 2
Armor piercing 10mm
ignores 10 DR
gun nut 2
explosive 10mm
high damage, lower accuracy
gun nut 3, explosives 2
explosive AP 10mm
ignores 10 RD explodes on contact
gun nut 4, explosives 2
The problem with this is ammo mods would require ammo working station and parts like powders, casings and materials. though most are in game like lead would be a huge requirement for ammo modding and crafting, but also copper gun powder, black powder and other materials would be needed.