A lot of them are specific to certain types of guns.
It's not very clear but it does list some specifics on the upgrade in small font.
It will say in big bold letters "Magazine Assault Rifle" and then smaller font "Extended Mag (50)"
And then in little tiny font it will say something like "7.62 x 39mm"
So the extended mag will only unlock for assault rifles that fire this type of ammunition.
There will be other weapon parts that unlock "Extended Mag" for other assault rifles that use other ammunition.
I assume they did this to make it harder for us to collect all the gun parts.