I'm sure the answers to this will vary quite a bit, but I'm curious, so I'll ask it anyway.
Some of you see a lot of shooters. Do you have a guess at how well the average hand gun shooter shoots at, for instance, a 25 yd target?
From my personal experience and observation at various ranges, what I would consider to be the "average" handgun shooter does not shoot well at all and would fail this test most assuredly. Most don't even shoot out to 25 years often enough to become proficient at that range, nor do they really seem to shoot enough to be any good at distances of 15 yards in. What I have normally seen is shooters shooting at targets btwn five to seven yards, and taking their time carefully aiming with all sorts of grips, mind you, spraying the targets with no discernible pattern or intended grouping as if they are pleased they hit the target at all. This suggests to me they have not had any training to know how to shoot proficiently, let alone know they need to be able to obtain a particular score of any kind.
What would you say an "average" 5 shot group size would be?
I would say that at a range of 25 yards the average handgun shooter will do well to hit the area center mass five times shooting a magazine of 15 rounds.
What would a "good" 5 shot group size be?
At 25 yards a "good" group size should be within 12 inches. Excellent would be within 6", and outstanding would be inside 3", IMO.
Answer for both offhand and from a rest.
For me, from the rest at 25 yards, taking my time, I should be able to put five shots inside 3", and this should be the same for either hand b/c you're resting the gun on a stationary balance. Offhand without a resting position, however, would be another matter. I would be happy to get five shots in the center mass in a magazine of 15 shots.
Specify firearm if you think it makes a big difference.
For the average shooter I definitely think the caliber and size of the gun will make a big difference. For an experienced shooter it will have an affect; however, I do believe there will be less of a difference for experienced shooters. Honestly, I believe the average handgun shooter probably shoots their gun maybe two to three times a year. The above average shooter probably will shoot at least once a quarter while what I consider to be a proficient shooter will shoot monthly. Those who shoot weekly will be noticeably more proficient and able to obtain much better scores.
Shooting is a perishable skill meaning a certain level of proficiency isn't something you can obtain and maintain without doing it often. It is very easy to tell experienced shooters from inexperienced shooters, but even beyond that it is easy to tell those who shoot weekly from those who don't. And, it is easy to tell those who train with moving targets, shifting from different positions, etc. from those who only shoot at the range on lanes. It's all a matter of the time one commits to it.