Stacking the smashers
 

Do the runebreaker/runesmasher things stack? Can I put 4 on a 4 armed chassis for massive penetration?

Re: Stacking the smashers
 

Two or more of the same item generally don't stack (An exception being bracers of protection)

Re: Stacking the smashers
 

No, same items do not add the bonus several times. You could addan item to misc slot though.
Generally true. There are however various items that do stack, like the supply-items (if you ever wanted to stack those on a toon) or water-breathing items (like Sea Kings goblets). I *think* the no-stacking rule is "only" relevant to path-boosters, penetration boosters and resistance boosters.

Re: Stacking the smashers
 

The no-stacking rule also applies to supply-items (As in two different bonuses do stack, but two or more of the same do not, unless you're referring to a different no-stacking rule?) Can't say about the water-breathing items, as I've never tested them. Assuming we're meaning stack as in applying the same bonus multiple times to a single unit, given that your two examples there could be interpreted as otherwise.

As a side-note, items that cast spells through-out battle will stack in that aspect if you equip multiple instances of them (i.e. 4x Scutata Volturnus on your 4-armed chassis is 4 instances of shocking-grasp, but only 50% lightning resistance)

Should also be noted that separate items that apply quickness (Jade armour/boots of quickness/Champions Trident, along with a W9 bless and presumably the spell quickness) will not stack in that area, however items that increase resistances (Alot.) will stack as long as each instance is from a different source (Brimstone Boots + Ring of Fire = +200% fire resist)

Re: Stacking the smashers
 

There are some oddities too, like Gate Cleaver + Wall shaker not stacking (just giving the gate cleaver bonus).

Best effect for (non-arti) penetration would be rune smasher [w2f2], void eye [s1] and spell focus [s1] (or RoS/RoW instead of focus if you need +paths), combining to total +5pen.

Re: Stacking the smashers
 

Note - this is for humanoid mage only. 4-armed figures generally have just 1 misc slot and some Pretenders and (iirc) others have more than 2 of them. Master Lich or Crone can have penetration +6/+7 not considering level boost.

Re: Stacking the smashers
 

On a related note: what spells work best when penetration boosted? Does penetration apply only to spells that must pass a MR check or also to common damage-dealing spells? Consider I'm playing LA Atlantis, so I can cast most mid-research water/death stuff like Falling Frost, Shadow blast etc. :)

Re: Stacking the smashers
 

Only mr check spells. That's what penetration does. Improves your mr roll.