This is going to be a bit oddball, but when assigning legacy weapons, we looked through the additional trats and stumbled over this:

"Shattering
This weapon is like an extension of its wielder’s body, channelling his strength into shattering blows, or splitting armour plates apart along invisible fault lines.
Melee: The weapon strikes with the Power Field Quality."

Now, this seems fairly clear, until you look at "power field" itself in the core rules:

"Power field:
A field of power flickers around weapons with this Quality,increasing its Damage and Penetration. Such modifiers are already included in the weapon’s profile."

When looking at the power weapons' stats, there seems to be no real common rule for by how much pen and damage increases, which poses a significant problem for anyone who wants to upgrade their legacy weapon with said quality.

My question here is:

Are there any errata or any rules from other FFG systems which stat out the exact increase in damage from power field?

There is no increase (apart from the automatic Legacy Weapon one) - if there was, it would say so in the rules. The Shattering effect merely means that you can shatter enemy weapons like a Force Fielded weapon can.

You can (technically) use power weapons with the powerfield off, though, admittedly, this isn't something most people do very often. Also, that lack of information makes it hard to turn non-powered weapons into power weapons.

I'd generally say +5-6 pen, +5-6 damage for most weaponizing powerfields, which, IIRC is about the difference for a common power sword to best-quality regular sword.

However, as regards the Shattering Legacy effect ... I'd say you just get the weapon breaking effect from Power Field.

On the other hand, in my games I give penalties for stealth test to anyone wielding a power weapon as it emits light and sound.

But if it gains the weapon-shattering power without actual power field, this should not apply. Also this means legacy weapon cannot be shattered by other weapon which is also good.

But if it gains the weapon-shattering power without actual power field, this should not apply. Also this means legacy weapon cannot be shattered by other weapon which is also good.

Except of course by weapons which explicitly state they can wreck power weapons such as Relic Blades from Deathwatch or other horrifically powered daemonic (some nasty corrupted nurgle blade perhaps) or xenos items (Phase Sword) and a few other certain things as and when the evil GM deems suitable...or maybe I'm just mean

But if it gains the weapon-shattering power without actual power field, this should not apply. Also this means legacy weapon cannot be shattered by other weapon which is also good.

Except of course by weapons which explicitly state they can wreck power weapons such as Relic Blades from Deathwatch or other horrifically powered daemonic (some nasty corrupted nurgle blade perhaps) or xenos items (Phase Sword) and a few other certain things as and when the evil GM deems suitable...or maybe I'm just mean

Well in that case

But if it gains the weapon-shattering power without actual power field, this should not apply. Also this means legacy weapon cannot be shattered by other weapon which is also good.

Except of course by weapons which explicitly state they can wreck power weapons such as Relic Blades from Deathwatch or other horrifically powered daemonic (some nasty corrupted nurgle blade perhaps) or xenos items (Phase Sword) and a few other certain things as and when the evil GM deems suitable...or maybe I'm just mean

Well in that case

If you're going to do it, do it properly. Say what you like about the Imperium they do a **** good job of cleansing a world from orbit.

So this is what me and my GM came up with:

You take a regular version of that weapon, and that's the unpowered version. So if you have a Shattering Sword, your sword is now a power sword for stats, but is still a primary melee weapon for proficiency. (use power blade for a Shattering Knife, and your best judgment for other more exotic weaponry).

In the case of a legacy weapon, well power-field has qualities above and beyond just damage and penetration, which I believe is more important, given the name of the upgrade, and that is the powerfield trait of damaging and destroying non-powerfield weaponry. Hence the "shattering".