So if I hit a 10-ton fighter with 15 armor with a 100-ton particle beam bay, which if I read the chart on page 24 correctly does damage code 9, and hit per step 5B on page 46 and penetrate on steps 5C and D, I roll damage in step 5E. Step 5E seems to tell me that I roll +15 for armor, and +6 for my weapon being factor 9 or less. That's a 21 already, so a 23 if I roll snake eyes, that means a 100-ton particle bay can't hurt a 10-ton fighter with armor 15. A pulse laser applies a -2, so snake eyes with a pulse laser scores damage once every 36 tries. But the automatic crit still doesn't apply because the largest pulse laser is rating 7, which is 7 auto crits on a size 0 craft, but 7 autocrits are negated by 15 armor. Larger ships won't even take the autocrit on a snake eyes hit. Nuclear weapons apply a -6 to the roll, so if you roll 6 or less on 2d6, you can get a hit, but 30 missile turrets are only rating 7 so no autocrits for 30 missile turrets. Luckily 50-ton and 100-ton missile bays can get to rating 9 and can get up to 2 rolls on the autocrit table against fighters, and even if half the crit table is meaningless for a fighter, it reduces armor by one. Now at 300 tons and up, size 3, those missile bay autocrits go away also (and no more autocrits from anything of size 9).
This is presuming your opponent has somehow gotten a hold of lots of nuclear missiles, which I understood were not handed out like candy at Halloween. I wouldn't expect pirates to have them, for instance. That means 15 armor means pirates without spinal mounts can't touch me, and I would expect spinals to be uncommon with pirates also?
So I am immune to 100-ton particle bay hits (and any other non-meson beam weapon, and non-nuclear missiles) if I max the armor, and nuclear missiles don't get it easy - half fail to hurt me, and the ones that hurt do pretty trivial damage. Why would anyone not max the armor at TL15? Why do I see so many warship designs that don't have max armor at TL15? I understand that at lower TLs armor weighs a lot more, and doesn't go up as high, so maybe isn't worthwhile, but at TL14/15, it seems it should be ubiquitous.