Hi Marcus,
Nice find, this one!
Are you sure, though, it actually consists of limestone? To me, its unusually smooth - and speckled! - surface rather points at a kind of Austrian
marble, known to have been used to form cannon balls: the so-called
Untersberger Marmor. It has been continually quarried at Untersberg/Salzburg, for about 2,000 years: from the Ancient Roman period up to the 21st century.
Apart from that, and from various excavations done on historic battleground sites, we have records of
pug! balls used with both arquebuses, muskets and pistols, and from the 16th through the 19th centuries.
All the limestone balls I have seen, including a specimen in
The Michael Trömner Collection 
(see attachment), show a markedly porous surface, which is characteristic for that material after being exposed to environmental and/or excavated conditions for five centuries. The surface of the limestone used for the architecture of Gothic cathedrals bears witness of the same phenomena.
Attached find images of characteristic limestone balls for guns.
The first measures ca. 40 mm in diameter, ca. 600 to 400 years old and most probably for a wall gun or a falconet, and was excavated in Regensburg, Bavaria, in 1879!
I also attached photos of
the only known limestone grenade, and retaining its original fuse!
All items in
The Michael Trömner Collection.
Please note that the scale is in cm.
For comparison, the last images depict five cannon balls made of
Untersberger marble (Dorotheum sale, Vienna, 19 Feb 2014, lots 37 and 36)!
All photos, except where noted, copyrighted by Michael Trömner.
Best,
Michl