and I believe the Danes too, were frequently issued single-ball and double-ball cartridges.
The idea was that if faced by cavalry, they were to fix bayonets and load the double-ball cartridges
In the danish army in the 1850ties 20% of the infantry had rifle muskets and everyone was issued a smoothbore.
(So yes, 20% had two guns. They used the smoothbore for drill, guard-duty, blank firing during field drills and similar peacetime activities. Saving the rifled guns for live firing and actual combat*)
For the smoothbore a buck and ball cartridge was used in addition to a normal one.
And I believe you are correct that it was suppose to be used against cavalry. In the field a soldier was suppose to have 55 ordinary cartridges and 5 buck and ball. (according to a 1855 book that was used for teaching NCOs)
If it was suppose to be used in a general firefight against enemy infantry only having 5 of them make little sense.
Having 20 or even 30 buck and ball would make more sense to me.
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* By 1863 all the smooth-bores had been rifled and the 1863 drill book, do have a note saying that during "dry" drill, the ramrods must not be used... to protect the rifling from wear. Only when loading blanks for field exercises or doing live firing are they used.
This also mean that the buck and ball cartridge was not in use during the war in 1864.
(and wrong decision in my opinion. As I see it most danish soldiers did most likely not have the marksman skills needed to hit at long range anyway and the terrain in warzone was full of hedges comparable to what most know from the fighting in Normandy in 1944.
So the combat range for most infantry fights was rather close range anyway. Firing buck and ball might have helped level the playing field against Prussian breechloaded rifles)