Reality-testing / Playtesting the Guns Default . . .

[weby]

I have taught the very basics of shooting to and lead though the first pistol shots for quite a high number(20-30?) of people over the years.

My experience is that about 1/3 just cannot do it well. Mostly it seems that they are too afraid of the gun. Very few of those are ever able/willing to overcome it.

About 1/3 have hard time hitting anything with the first few dozen up to a hundred shots. Mostly it is a question of shooting too early, having hard time understanding the sight picture, pulling trigger too hard or similar things and some people just get frustrated from such and thus take longer to adjust. They can eventually learn to overcome such things though.

About 1/3 start to hit reasonably after few shots. That is once they are no longer "surprised" by the gun, something that tends to happen to anyone the first few shots. And then there was the one who had never shot before and shot 5 shots in a 5cm group at 10 meters as first shots ever...

But then to the actual test you did and modifiers:
In reality at least the need to know exact range depends on the drop of the weapon in question and thus at short ranges is more important for something thrown that thus arcs quite a lot.
For a 9mm pistol the difference in drop is negligible at that short ranges. In fact out to about 30-40 meters most shooters do not have to factor it in as the difference is so much smaller than the group size. So if a target is at 10 yards or 20 it does not matter unless doing professional level shooting. But the same 10 yards is a huge difference at 100 yards, difference between say 90 and 100 meters might be 7-8 cm drop.
So basically I find the range finding rules in tactical shooting a too much of generalization for closer ranges and even on longer ranges the requirements for getting +3 or +1 are kind of silly. Basically a +1 means you hit at 1.5 times the range and +3 you hit at 3 times the range, to have a drop of 2 times if the target moves few feet...

So my guess at modifiers:
9-4=5 base skill.
Likely +3/no risk/stake in thing total as you do not seem to fall into the first group that would perceive it a risk to self/others. Or the second group that places too high stake on it.
Likely +3 for range, and definitely at least the +1 by the rules(see my rant above)
likely +2 or +3 for the conditions as you would not have the wind, variable lighting and such of the +1 outdoor range, but you did not have the perfect range +4 either due to the light.
You likely braced the gun in 2 hands: +1
Acc +2 for the glock.
All out attack +1
Most people do take the extra aim time when on the range even if they do not think about it, so +1 or +2.

Thus the total without target size/range would come to: 16-20
Range: 15 =-5
Size of target: If you had a 8-12cm diameter target that would be the -5 (>3 inch sphere): -5
So skill 6-10: resulting in really none to half the shots hitting that size target, but most people use larger target sizes, like the 30cm diameter targets I tend to use for first time shooters(-3): such would give skill 8-12 and the upper range would thus well correspond to your experience.

So back to reality in my experience:
I tend to have people shoot the first shots at about 10 meters at a target with 30cm "black zone", giving 4 range penalty and -3 size penalty in Gurps terms. And most people tend to hit with most of the shots on the black after their individual adjustment period (few shots for group 3, more shots but still usually achievable in one or two sessions for second group).

As a side note: Rifle shooting seems to be slightly easier to grasp for people and the difference in early hitting probabilities is higher than the 3 higher accuracy bonus, with the hit probabilities at the 50m range on slightly bigger(40cm) targets tending to be higher, despite nominally being the same difficulty(3 higher acc +1 larger target, -4 more range).