Some hammer springs are lighter impact and need to be changed to the full 223, This is mainly because the 5.56mm military primers on some ammo are harder than others so some have strong springs in them. Try it without or just go and change it, its only a few bucks.
Or just throw some Euro ammo through it and see how it fires. Some Geco ot Dynamit Nobel, if not Fiocchi. Euro primers are harder as well and thats the main reason that they went to inertia firing pins later than we did.