I have a modern fantasy pirate setting. Would modern spearguns be good weapons to use?
So, spear guns are a hunting tool. They have to be reloaded after each shot, and (depending on the spear gun) that can be an involved processes.
Spear guns are not designed for use out of water. Some cannot fire at all, and others will not react well. With band driven spear guns, the lack of water can cause the mechanism to fail, throwing the band back in the user’s face.
I don’t know what your max range above water would look like, but underwater, long range spear guns have max ranges somewhere under 10m. That might sound reasonable, but remember that most (decent quality) handguns and shotguns can reliably put someone down at 50m.
If the spear connects, that can kill someone, however that requires hitting them. If you’re underwater, the spear gun becomes a potential stealth weapon. If you’re above water, it’s not an option.
If you’re patterning off the modern world, you might want to look at what real world pirates use today. I don’t simply mean, “AK pattern rifles, and other ex-Soviet hardware that’s floating around.”
The Cold War, and the resulting proxy wars meant there was a lot of very durable military hardware circulating in countries that saw conflict. In the real world this is a very eclectic mix. As assault and battle rifles were the primary infantry weapons, these are the most prevalent. A pirate today could be using a rifle that was originally manufactured over half a century ago.
I’m not sure that an international bipolar structure is necessarily inevitable. We’re certainly moving beyond that into a multi-polar system now, and even during the height of The Cold War there were a lot of distinct international players, even if Soviet and NATO leaders wanted to view the world in a harsh black and white.
With that said, warfare is inevitable. The single factor that, most heavily, averts warfare is trade. When your economy is partially dependent on the actions of another nation, the last thing you want to do is pick a fight. (I’m oversimplifying slightly here.)
Even in the event that you have an (otherwise) peaceful world, you’ll still see a need for militaries, to deal with the disaffected. These are forces which are too well equipped and hostile for normal law enforcement responses. The cliché answer here are terrorists, but pirates are another case, one we don’t often think about.
A decade ago, the modern pirate was viewed as Somali by default. Somalia’s government had complete collapsed by the early 90s, leaving Mogadishu as one of the only real examples of a feral city.
Alone, a failed state wouldn’t have lead to pirates, but off the north coast of Somalia is The Gulf of Aden. Any shipping moving from the Atlantic or Mediterranean to the Indian Ocean must either pass through the Gulf of Aden, or around South Africa. (3,750 miles away.) Roughly 12% of all seagoing shipping passes through The Gulf of Aden.
The lack of a functioning government in Somalia, combined with rich shipping just off the coast, was a perfect recipe for piracy. And it’s happened more than once. In the second century CE, the Roman Empire deployed a permanent naval garrison to the Gulf of Aden to deal with pirates. This isn’t critique of the people, simply a consequence of the geography creating a natural choke point.
It’s worth noting, before we move on, that the Somali pirate is already (mostly) history. A coalition government was formed in 2008, and it’s been slowly rebuilding the country. Additionally, the entire region has seen a dramatic increase in naval patrols (from a number of countries.) Combined, it’s dramatically reduced regional piracy.
Now, let’s take that model for a second and apply it to the golden age of piracy. Economic exploitation of The New World was driven by European empires. The European powers were warring with each other, meaning the bulk of their navies were needed at home. (Both as defense and deterrence.) Shipping from South and Central America needed to pass through the Caribbean (the entire point was to move foreign goods to consumers in Europe), and military protection was limited. The result was nearly a century of piracy.
A weird quirk of the mercantilist economics practiced in Europe at the time contrasts hard with modern globalization. The European empires were trying to set up entire, self-sufficient, economies in parallel to one another, selling the final product to whomever would pay. This put them in conflict. In contrast, modern economics tend to run supply chains across national boundaries. (The is where the economic interdependence reducing wars comes into play, not simply, “we trade with them so we don’t want to go to war.”)
So, what do you need for a pirate infestation?
Having an area that is difficult to patrol, is a must. Failed states (and feral cities) are almost a necessity today (because civilization of one form or another basically blankets the planet, especially anywhere we take trade goods.) The Caribbean is far less attractive to piracy because of its proximity to a major naval power (something that was not true during The Golden Age of Piracy.)
A background war that ties up the major navies would be advantageous, but not necessary. (Though I doubt the average pirate would have the background in international politics to understand this point, and actively foment the war.)
Also relevant to the Golden Age of Piracy, and less so to modern Somalia, having wars going on in the background will result in experienced sailors who no longer have a job when hostilities cease. At that point, piracy (or privateering, (if letters of marque are being handed out) will be one of the few ways they continue to earn a living, while still using their skill set.
Depending on how different your modern fantasy world is from the real modern world, you could expect to see a lot of infantry hardware, and possibly even decommissioned military ships converted into pirate service.
The end result is, no, your modern fantasy pirates probably wouldn’t want to use spear guns. They’d probably be using normal guns. Rifles and shotguns, with the occasional handgun.