In this old thread in the Beta-forums, the questions was asked:

Does it apply to a shot where the total modifier is positive? For example, making a single shot (+10) at long range (-10) after having aimed for a half action (+10)? There are penalties, but they're outweighed by the bonuses.

Was this ever addressed?

Will this piece of gear do nothing when firing a full-auto weapon (-10) on Short Range (+10), but suddenly kick in at Normal range, when the total penalty to the shot exceeds 0?

As the wording says "all penalties" I am inclined to say that as long as there are any penalites in the equation, whether from range or firing mode, they are reduced before calculating the net total. Otherwise it becomes kinda strange, where the Targeter only "works" sometimes, depending on other circumstances.

That is my interpretation. You figure out the total penalties, subtract 10 from it, then add bonuses for your total.

Actually a targeter only reduces the difficulty of a shot if the total modifier after all modifiers are added is negative.
The idea being that targeters cannot assist with already easy shots, but can help mitigate the difficulty on really tricky shots. This is why they are mainly issued to Stormtroopers and dedicated marksmen and not all line troopers.

Kasatka said:

It specifically says it reduces all penalites on ballistic skill tests by 10.

This can have 3 interpretations.

1. Only subtract 10 if once all bonuses/penalties are totaled the result is a penalty.
2. Substract 10 from the total penalties before adding bonuses.
3. Substract 10 from each individual penalty that applies to the ballistic skill test.

I choose to interpret it as option 2 as it's a rare piece of equipment to aquire so I'd like it to apply to a wide variety of situations without being extremely over/underpowered.

Unless there is an errata to clarify there is an argument to be made for all 3. From a completely RAW point of view I'd go with 3, but as a GM I'll stick with 2.

Considering how often FFG just copy + paste from previous lines i am very surprised that upon some investigation they didn't just use the errata'd Targeter from Dark Heresy:
http://www.fantasyflightgames.com/ffg_content/dark-heresy/pdf/darkheresy-errata-v3.0-printable.pdf

"The description of the Targeter on page 179 should have the last two lines removed and replaced with “A targeter grants a +10 bonus on all Ballistic Skill Tests made with the weapon.”"

In all honesty that is the most sensible way of treating a Rare upgrade to a weapon, basically +10 to BS when firing it

But then the original wording of the Targeter in Dark Heresy was to grant +10 to semi/full auto I believe, so it was always a different beast than it is in Only War.

But I agree, keeping a flat +10 to all BS tests, and counting it as a sight (to avoid stacking with Red Dot, etc) seems fine. Melee weapons can gain +10 from craftsmanship, so this upgrade is unlikely to unbalance anything.

I don't see the problem with the literal wording.

Firing at Extreme range becomes -20 instead of -30. Firing through smoke (say) becomes -10 rather than -20. Firing at a prone target becomes +0 rather than -10.

So firing at Extreme Range through smoke at a prone target is -30 rather than -60. What's the problem?

bogi_khaosa said:

The terms are not quite written like that. The term "all penalties" is a collective term for a sum which is then reduced, while your version would certainly be correct if it said "each penalty". Your take also makes the targeter very powerful.

HappyDaze said:

Indeed - very powerful but only when dealing with odds stacked against you. You'll see situations where a marksman with a targeter on his weapon is hitting as accurately as someone without one, simply by always shooting at slightly concealed targets, at longer ranges, through slight weather effects - heck even while fatigued!
So no, it should be +10 bonus after ALL penalties are computed, OR a flat +10 BS to the weapon. I favour the latter as it justifies the rarity of targeters, though the former is closer to the RAW i believe.

Droma said:

I'm with Droma on option 2 as noted from the quote above. The targeter reduces any penality by 10 - not range specific, no restrictions for light, an enhances (by defult) any difficult (aimed/cover) shot.

I'm going with option 2. Giving a it a +10 BS doesn't seem to be the intent at all.