Okay, I don't want to sound like a total whiner but I have to point this out: bombers lack protection. Let's look at the armors available for every weapon class.
Swordsmen
Default: Skolver, Vog Cub
Shadow Lair: Snarbolax, Heavenly Iron
Total: 4 sets, with at least 1 set for protection against each of the special damage types
Gunners
Default: Justifier, Nameless, Shadowsun, Deadshot
Shadow Lair: none
Total: 4 sets, with at least 1 set for protection against each of the special damage types
Bombers
Default: Volcanic, Bombastic, Mad Bomber
Shadow Lair: Mercurial Demo
Total: 4 sets, with everything protecting against elemental and no sets for shadow or piercing protection
…
If anyone can give me a reasonable explanation of why bombers have no defense against two of the three damage types, go ahead.
I don't know, but I'll point out two things.
First, gunners also have Perfect Mask of Seerus (just to strengthen your complaint a bit ;).
It seems that, when Spiral Knights was designed, normal and elemental were definitely the "main" damage types. For example,
* slimes didn't do piercing damage
* there were no piercing swords or (pure) piercing bombs
* there were only one shadow sword and one shadow gun, and no shadow bombs.
If you wanted to do piercing damage to a beast or fiend, you had to use a gun. In contrast, there were several elemental guns and bombs, and two elemental swords, and tons of normal weapons. Over time, Three Rings has added more and more options. Perhaps they just haven't gotten around to non-elemental bomber armor yet?