Imagine that I have the skill "assault rifles" at 1 and the skill "shotgun" at 10.
Now I want to fire with an Ares HVAR assault rifle.
If I use the skill "Assault rifles" in this case I could use only 1 dice + the whole combat pool OR I could use my "shotgun" skill using 10 dices + 5 dices of combat pool (half the base skill) with a target number +2 than the normal.
In case like this (I admit, it is not very common) I think that is more profitable to use the default skill than the proper skill: Is it possible to do it or not??????

Perhaps add half your shottie skill to the AR roll, with a +1 TN?

I wonder how the math works out.

ain't plastic warriors got a good skill web and rules about defaulting inside of it?
try this http://plastic.dumpshock.com/shadowrun/skillweb.pdf

ABout BlackSmith answer:
Ok, I'm thinking about it .... although this is more a homerule based on the 2nd edition than a 3rd editio ncanon anwer
About Dr.Pain answer:
Is it canon to use active sckills as complementary skill as you suggest?
and furthermore:
is it canon to use a anyway a default skill (although it is good it is anyway a default skill) when you have already a (shit of) proper base skill?

If you have the appropriate skill you cannot choose to default instead. In this case youd simply have to use your assault rifles skill of 1, and up to 1 combat pool.

While you are rolling a lot less dice, you do get to use a better TN. Also, because low level skills are pretty cheap karma wise its not too difficult to get a skill of 2 or 3 in quick succession, minimizing the times youre limited to so few dice.

It has come up in our games that sometimes it seems like youd be better off without the skill (trolls of body 12 wishing they could use it instead of their atheltics 2 skill) but really, you get a better statistical advantage with a lower TN most of the time. Sometimes it just feels worse to be rolling less dice, but really when you look at the odds its not bad at all.

Your particluar exmample is one where using more dice might actaully be better for the player (2 dice at base tn, or 15 dice at base tn plus 2), without a specific situation to apply mods from its hard to say. For reference, assuming it was short range with no other modifiers it would be on average 1 success compared to 2, a small difference. If it was short range against a target in cover, its rougly 50% chance of 1 success compared to 50% chance of 1 success, which is the same.

So while the values look to be a big change its really rather irrelevent. And this is comparing a huge skill desparity. If the assault rifles was skill 2, allowing 2 combat pool, the whole comparison would favor the actual skill every time.

There used to be a half-default rule (+3 TN) where the skill existed but was poor ...?

In any case, within a family of skills (+2 TN default), my vote would be to use the default skill instead. Any more penalty to TN than that, and I'd use the skill as is.

Don't forget about limits on defaulting: If a skill requires a test with a Modified Target Number of 8 or higher BEFORE the Defaulting Modifier is applied, a character cannot default to that skill.

So given an average scenario of low light, you can't attempt anything over medium range (over 20 M). If your target has partial cover (+4 Mod), you can't even attempt short range.

Once you start a skill, even if it's at one, you have to use it. The smart thing to do is train long enough to start with the skill at 2, and always use combat pool with it to avoid the rule of ones.

I miss the 2nd ed skill rules.

Except for the existance of knowledge skills I dislike everything that changed in the new one, and some of what stayed the same (eg the athletics skills ridiculous breadth).

Just for clarity, heres the defaulting rules:
-You can default to a skill if theyre shared in a box on the skill list, with a +2 tn mod, and can use half the normal amount of pool.
-You can default to the specialization of a boxed skill (to presumably get more dice), but the tn rises to +3, and can use half the normal pool.
-Lastly, you can default to the linked attribute, with a steep +4 tn, and no pool allowed.
-If youre defaulting in any way, you cannot default on an action with a modified tn of 8 or higher (before the defaulting tn is applied) because its just too hard to do without any training in the matter.

Combine this with my example above, and youll see why having even a small skill is better than defaulting almost everytime. Not only can you always attempt a task, but statistically youre not really losing much if anything at all in terms of chance of success.

edit: Shadowghost is essentially correct. If you have the time training from 0 to 2 is the best option, but even if you cant simply staying at 1 for a single adventure isnt that bad. The key is to just get a few dice (2-4) in any skill you need to have but not be a master of, in order to have a fair shot at most rolls. Defaulting is ok for awhile, but lower level skills are cheap enough that its worth the small karma and time spent getting them for the advantages mentioned.

ok, i understand your opinion but I would like to ask you another thing.
Is it possible to use an active skill as a complementary skill for another active skill? ?
For example if I want to fire with my "rifles" skill 6, can I use as a complementary skill my "Shotgun" skill 4, allowing me to have another 2 dices to throw??
I think it is not possible but there was this problem in my group.

In a word, no. Complimentary skills are with GM jurisdiction and 95% of the time involve knowledge skills. An example might be allowing a players pychology knowledge to compliment negotiation, or allowing a players engineering knowledge to compliment a demolitions check to level the building.

Also, again for clarity, if you were to use a complimentary skill, they do *not* add half their rating as dice, they add half their successes. Big difference.

Back to the example, someone with negotiations 6 attempting to use psychology 6 as complimentary does NOT roll 9 dice. They roll their complimentary dice at the base skill's tn. Then they count HALF those successes (round down) on the real skill test, assuming it nets at least one success on its own. So in this example theyd roll psychology 6 at tn 4, netting 3 successes. Then theyd roll negotiation at tn 4, and as long as they got 1 success on the real negotiations test they could count half round down (1 in this case) more successes. If the real test gets no successes, the complimentary ones are lost as well.

Defaulting to the attribute gets a bit silly when you're talking about Vehicle skills and you have your ultra-Reaction physad. "OK, so I don't have any idea how to fly an LAV vectored-thrust combat aircraft. But I'm rolling a mere fourteen dice against a TN of 8... chances are I'll get at least two successes..."

I think any time one is defaulting to an Attribute there should be a threshold to accomplish anything.

i miss the firearms skill

IIRCR adept reaction increases dont apply on rigging or driving in any way. (i just might RC wroong too...)

and i love the 3rd ed. rules.
all of them.
skills, magic, matrix.... all.

they are 10meters better than the 2nd edition and 3,765km better than the first edition.

WotC should take excample from SR how updating is done...

One very important thing to note about using a crap skill vs. using a much higher default skill: The maximum number of successes. There are lots of tests where the number of successes is all that matters, not whether you get any successes at all. If you only have 2 dice (1 skill, 1 CP) in pistols, you will not be killing the person you're shooting. If you use 9 dice (6 skill, 3 CP) from defaulting to some other skill, even if the TN goes from 4 to 6, you still need those possible extra successes to have a chance. This also becomes more useful if you have KP to spare.