What does sclopētum mean in Latin?

sclopētum, a noun, means “rifle; gun; firearm (in general)”.

What is the genitive of sclopētum?

The genitive singular is sclopētī. sclopētum is neuter, second declension.

In classical Latin, [skɫɔˈpeː.tũː]; in ecclesiastical (church) Latin, [skloˈpɛː.tum].

noun · neutersecond declension

sclopētum n (genitive sclopētī)

Classical[skɫɔˈpeː.tũː]Ecclesiastical[skloˈpɛː.tum]

New Latinrifle
New Latingun; firearm (in general)
Interim rumor vagatur... ita esse Mediolanensem aciem instructam, ut singuli singula gererent Sclopeta, genus sane tormentorum terribile admodum atque exitiale.Meanwhile a rumor spreads... that the Milanese army was equipped in such a way that each soldier carries a single firearm, a type of weapon that is truly very terrifying and deadly.1732, Lodovico Antonio Muratori, Rerum Italicarum scriptores ab anno aerae christianae quingentesimo ad millesimumquingentesimum, volume 21, page 296:
Janizarii præterea et hoc elogii bellici merentur, sua Sclopeta, bombardas, etc.Moreover, the Janissaries also deserve this military distinction for their firearms, cannons, etc.1724, Johann Peter von Ludewi, Reliquiae manuscriptorum omnis aevi diplomatum ac monumentorum, ineditorum adhuc, volume 6, page 311:

From (onomatopeia) scloppus (“bang!”) + -ētum (alternative form of -ittus/-ittum).