The NATO rounds were introduced in the 50's and since then most military weapons have been made to use those sizes
With pistols and smgs the standard seems to be between the 9mm parabellum or the .45 acp
So that's, what 4 or 5 types of bullets, making generalised ammo quite realistic
But for there to be truly one kind of universal ammo you have to go energy weapons, heavier weapons would of course have some quite heavy capacitors.
Of course the developers need to balance the weapons so there is no obvious choice for every situation.
It could be trough consumption, as the balance between killtime and % of the energy cell used to fire the weapon, make the weapon with the lowest killtime also use the most ammo per kill.
This can be used to give the player a choice of playstyle instead of forcing the player to use a weapon that they do not find interesting.
for those who have no self control and run out of ammo halfway trough the level you could introduce a weapon with regenerating ammo like the marauder rifle in starship troopers or the bug gun (forget the name) in half life
If you absolutely must have the player use different weapons, you could just add more variation to the enemies instead, give them resistances or something
Another thing that could be explored with universal ammo is universal guns.
The best example I have is the gunman game, where most guns had a dial or two to customise them.
I would also point out that universal ammo is no more immersion-breaking than say the vacuum in the trouser leg, regenerating health or selfilling mags
With pistols and smgs the standard seems to be between the 9mm parabellum or the .45 acp
So that's, what 4 or 5 types of bullets, making generalised ammo quite realistic
But for there to be truly one kind of universal ammo you have to go energy weapons, heavier weapons would of course have some quite heavy capacitors.
Of course the developers need to balance the weapons so there is no obvious choice for every situation.
It could be trough consumption, as the balance between killtime and % of the energy cell used to fire the weapon, make the weapon with the lowest killtime also use the most ammo per kill.
This can be used to give the player a choice of playstyle instead of forcing the player to use a weapon that they do not find interesting.
for those who have no self control and run out of ammo halfway trough the level you could introduce a weapon with regenerating ammo like the marauder rifle in starship troopers or the bug gun (forget the name) in half life
If you absolutely must have the player use different weapons, you could just add more variation to the enemies instead, give them resistances or something
Another thing that could be explored with universal ammo is universal guns.
The best example I have is the gunman game, where most guns had a dial or two to customise them.
I would also point out that universal ammo is no more immersion-breaking than say the vacuum in the trouser leg, regenerating health or selfilling mags