Naval weaponry and autocannons., Am I insane or are they absurdly good? Options

[Smobey]

I'm trying to run a naval-focused Shadowrun campaign about piracy and smuggling, and running some numbers against a hypothetical 20 armour / 14 body ship, checking out how much damage they would do. So here's what I figured out.

A heavy machine gun wouldn't really penetrate the armour, it'd only be effective against really light boats.
a LAW would do 3.67 damage on average. Makes sense, nice disposable weapons to give to mooks, low threat but still capable of doing damage.
A torpedo would do 7 damage. Two of them would sink the vessel, very reasonable.
Mercury Ship Laser would do 9 damage. They're one million nuyen beasts, so they should be good.
Vanquisher heavy autocannon: 16.67 damage. Whoa, whoa, what?

The 11P base damage, -6 AP would mean that any decently anti-armour shells would pretty much automatically penetrate the armour, and the insane +14 DV from the full autofire means that the vanquisher should sink every single other naval vessel (except the ones that use structure instead of body) from a single hit, easily.

I understand that it's 20F, but that's still gettable with the Restricted Gear quality. And torpedoes and ship lasers and such are far harder to get than that. In fact, anything that IS within the normal 12F restriction range would be completely ineffective against even moderately armoured boats, completely incapable of piercing through the armour. So giving them an access to restricted gear is a must.

To make matters worse, we've already played a few sessions, and it's a little too late to simply forbid them from taking it. So it sort of looks like I'm going to have to nerf it hard, or every single naval battle ever will end instantly someone hits the other with a narrow burst. Unless I'm misunderstanding something horribly wrong.

Should the naval combat in Shadowrun between smaller vehicles really be that deadly? It seems like it'd make the multi-million corvette class vehicles completely worthless, since any old boat capable of fitting a heavy autocannon (or hell, a helicopter) would automatically blow it to bits from the first hit. Though sure, the corvette could have a dozen such guns, being capable of blowing up twelve other corvettes within the first few seconds of combat...

I do realise that the autocannon has a far inferior range compared to torpedoes and such, and if the players really start abusing the fact they can automatically destroy any non-structure vehicle in the game with a single burst, I can start abusing the fact that I can attack them from twenty kilometres outside their range. And I get the weird feeling that that's when the game stops being fun.

So, am I being reasonable in wanting to tone the autocannons down? If so, by how much? Does anyone have any effective houserules for smaller-scale naval combat?