According to Jim Bob, from this thread , half of all Australian Army recruits can, without practice or previous firearms' experience, and after a single days' firing, achieve the minimum standard, which is a six inch group at 220 yards, firing a Steyr assault rifle, prone, supported, in their own time.

Since half of them can do it, in game terms that translates to an effective skill of 10. So, after adding up all the bonuses and penalties to their skill, we have to end up with 10.

Luther, later in the thread, compiled all the remarks and fixes made on the original post. First we'll list the modifiers that apply to this action, and then we'll consider the stats and skills of those recruits.

220 yards is -12 or -13, according to how you round (up or down) (p.B550). Here, we'll assume -12.
Target size is 6 inches, which is -6 (p.B550).
The target is a circular shape, so it's also +2 (p.B550).
A Steyr has an accuracy of +5 (p.B281).
A Steyr also has a scope of x1.5, for an additionnal +1 (p.B289). Here I've rounded to x2.
The rookie can aim as long as he wants so spending 2 additionnal seconds aiming, he can get +2.
The rookie has his weapon braced (+1).
This is an All-Out-Attack (Determined), for +1.
It's also an Easy Task (+5), because the situation is not stressful (which is not the default assumption for combat tasks such as shooting!).

Total modifiers = -12-6+2+5+1+2+1+1+5 = -1

Note Jim Bob#comments: The target for Australian Army soldiers is not a round target (bullseye) but a "Figure 11" target, which is a yellow oblong with a black figure of a charging soldier drawn on it. So that eliminates the +2, and brings the modifiers to -3. So the effective skill is 7, not 9, greatly changing the probabilities. Luther#comments: the "six inch group" mentioned by Jim reminds me a round area, though.

Note The "six inch group" probably refers to three shots hitting the target such that they can be surounded by a six inch diameter circle. The Figure 11 is probably about the man sized.

According to Kromm, DX 10 is the average value for a person suitable for military service. After 1 point worth of training in Guns(Rifle), the average skill score is therefore 10, modified with -1 as explained above. This gives our rookie an effective skill of 9, a bit below the expected 10.

In his post, Luther also takes into account the fact that people entering a professional military training are bound to be slightly better than people that are just suitable for military service. Playing around with statistics, he states that the average rookie's DX score must be almost 10.4 instead of 10, which gives a 42% chance of hitting the target and succeeding the test (1 cp invested and -1 penalty, for an effective skill of 9.4). For complete details, see his post.

Note I'm not sure that 1 point spent in Guns(Rifle) is very fair. 1 point is roughly 200 hours of study, or a week or two of intensive military training. AFAIK, only the "Quick Learning Under Pressure" rules would allow an absolutely untrained rookie to get 1 point in that skill... Without this, he would only have DX-4! Jim Bob#comments: Agreed. DX-4, and the "-3" modifier total, gives us effective skill of 3. Only one in 216 shots will hit the target area, a performance 113 times worse than that actually achieved in practice. Luther#comments: I've to disagree. As Kromm explained 1 cp is in the order of 200 hours of study, more likely to be 60-600 hours. I think 1 cp reflects well this basic training and, as showed on the forums, the GURPS figures are a bit pessimistic but not way too far from reality.

Conclusion

With this example, GURPS rules look fairly realistic, although a bit pessimistic. In fact, they were designed for stressful situations (fights!), and fail to correctly model calm, everyday situations.

Such situations are quite rare in a game. However, should one arise, the GM's going to have to take into account all parameters, in order not to get unrealistic rolls. For example, a common mistake with the Australian rookie shooting test is to forget the +2 for the round shape, and the +1 for the All-Out-Attack. This makes the total modifier -4, against which most rookie would fail.

Unless the task is very important to the scenario, without being stressful (this would be a very weird situation!), a good idea is to treat it without applying the official rules. For example, if 2 characters are challenging each other in a shooting contest, using the previous shooting configuration, there's no need to apply all the modifiers - a Quick Contest of Guns(Rifle) will be enough. The GM can also apply abitrary modifiers (after all, the situtation is not that important).