Roman Wars 270-500 AD
A friend asked if the location of the extant finds of Plumbata coincided with army on army battle locations that Rome was involved in from the era that the finds were dated from. In comparing a timeline of Roman wars with Plumbata find locations, here is what I found. A note, there were many small skirmishes and conflicts which took place in and around Roman held territories which I have not taken into account for this post since the question focused on army on army wars.
When Diocletian was named emperor in ad 284 there already existed 2 legions noted for their use of the Plumbata who were stationed in Illyricum where some of the earliest finds were located. These were dated around 270 ad so I began my search here. In 279 Probus launched a campaign against the Vandals in Illyricum. It seems probable that the legions used Plumbata in these battles and their fame for their use began here. In 285, in the region of the Morave River east of Illyricum (present day Serbia), Diocletian defeated Carinus. Here again, it is most likely that Diocletian would have used his best legions (which he had named Jovian and Herculian a year earlier for their skill with Plumbata) as his primary troops. In both of these regions Plumbata have been found.
There have been several Plumbata finds in Britain where, between 286 and 296 the Carausian revolt took place. Many have also been found in Germany where, in 357, the Battle of Strasbourg took place between a coalition of Germanic tribes known as the Alemanni and Emperor Julian. Others have been found throughout much of Southern Europe where Roman troops were stationed during this time.
It is worth noting that most of the Plumbata found have been at the sites of Roman fortifications. These were built in areas where conflict was either expected or taking place and then abandoned when the conflicts were resolved. So, to answer my friend’s question I would have to say that there is definitely a direct correlation between areas where major battles took place and where Plumbata have been found. My research also seems to indicate that Plumbata use spread from the region of Illyricum outward to the rest of the Roman Empire based on the dating of the finds.
The website Roman Army Talk has a list of extant Plumbata heads (from page 44 in the Plumbata thread, moderated by Robert Vermaat): Currently there are 175 published finds: 31 from Serbia, 30 from Britain, 16 from Slovenia, 15 from Italy, 15 from Austria, 14 from France, 10 from Hungary, 9 from Croatia, 7 from Germany, 7 from Switzerland, 5 from Georgia/Abchasia, 4 from Romania, 3 from Bulgaria, 3 from Greece, 2 from Liechtenstein, 2 from The Netherlands, 1 from Belgium, and 1 from Slovakia. Added are 88 from doubtful or unprovenanced origins (up from 80) for a total 263.