At 10 yards, slow fire, center fire handgun. Three inch group, two inch group, one inch group?
That depends on what you are shooting, a 2" barrel or 6" barrel, hot loads or subsonics.
I consider "good shooting" to be clearly above-average. For a service-grade centerfire autoloading handgun (Glock 17/22, Beretta 92, SIGs, etc.) with a favored/accurate ammunition selection, I'd say under 3 inches @ 10 yards, and under 2 inches for the longer-barreled guns (most of my Glocks will do 2"@10 yds, if I'm up to the challenge on a given day). For a 6-inch barrel or longer revolver with accurate/target ammo, under 2 inches would be needed to make "good", and many accurate revolvers in the right hands would do an inch. You didn't say position (unless "slow fire" indicated NRA one-hand bullseye-type slow fire shooting). The measurements I listed would be for a two-hand, unsupported standing position (something like Isosceles or Weaver).
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)
Is the person assaulting me dead?
That's good shooting
For target shooting, in slow fire, at 10 yards ANY gun should be able to make one ragged hole if the shooter knows what he's doing.
3" works if center mass. One thing I do is practice at 25 yards alot with my .22 . I try to run 300-500 rounds a month through it along with practicing with my other pistols.
I used to have a stainless steel Gold Cup (full sized 1911 in .45 caliber), and I always shot that gun exceptionally well. It pointed naturally for me, the recoil was not heavy (probably due to the weight of the pistol) and it recoiled straight back (instead of flipping the gun up). It had target sights and a long sight radius. And it had a wonderful, light and smooth single action trigger. All of which conspire to make shooting accurately fairly easy.
Every once in a while I would shoot better than normal (with normal being about 3" at 50 feet). One day I shot two targets (7 rounds) at 50 feet making a single ragged hole in the center of both bullseyes that you could cover with a quarter.
I wrote on the targets "50 feet, full-house loads, two hand hold, 7 rounds". I intended to show them to a friend for bragging rights. I stopped for lunch on the way home. Both targets were on the passenger seat.
When I got to the car I saw that the meter had expired and there was a cop at the car. I said, "Are you going to write me up?" and he replied, "Nope. Not going to ticket someone who can shoot like that."
I never did figure out if it was fear or respect that kept me from getting the ticket.
But to answer the O.P. question. "What is good shooting?", and the answer is shooting that is good enough to keep you from getting a parking ticket.
There are a lot of factors in good shooting. For carry concealed, shooting from 20 feet is a good "across-the-room" shooting distance. As long as you can reliably keep the rounds within the center of mass at that distance you are doing fine.
Jurisdictions vary, but on Long Island, where I used to live, if you fired or exchanged fire at distances much over 20 feet the cops and the DA would consider that a "gun fight" and they seriously frown on gun fights. The presumption is that at greater distances you can find cover from the other shooter and since you are "safe" you should not return fire. Tactically this makes sense too. If you don't return fire then the bad guy will feel confident to approach. Once he is within the 20 foot range you are golden and you can open fire.
So 20 feet is a good working distance to shoot from. Above I described the Gold Cup, a nearly perfect gun for accuracy, but a very imperfect gun for concealment. So depending upon how far your differ from the Gold Cup and how close you approach a snub nosed revolver, will determine how easily you can shoot accurately.
In most cases if you are threatened directly you will be shooting at less than 10 feet. If you are protecting a 3 party, then the distance can grow. And obviously the 20 foot criteria can be tossed out the window if you are protecting others.
But initially I would train at 20 feet and aim to get all the rounds in the vital zone. I always practiced a "tap-tap-tap". A three shot burst with two to the chest and a single shot to the head. I would lower the gun and repeat. Count your rounds as you shoot. You should always know how many shots you have left. In my opinion your slide should never be locked back. Drop the mag with the last round in the chamber and slap in a new magazine.
The same for revolvers. You should never pull the trigger on a spent round.
There is a lot more than accuracy involved in defensive shooting. You need to have situational awareness (who is around you and what are they doing?); a knowledge of when to draw and when not to. And the ability to be calm and steady under pressure.
For reasons that escape me I become calmer and steadier as the pressure rises and then I "fall apart" steadiness-wise once the pressure is off and the danger is passed. Everyone is different so you need to know yourself pretty well too.
The better part of 10 years with several different pistols. I've got a vid of a 1911 that's just a touch slower than the G32 as well. The only one of my pistols I can't do it consistently with is the HK USP. I tend to over do it on the safety and decock the damn thing during the presentation. That adds some time for sure.
YouTube - Kimber Raptor Pro
To the gentleman with the USP/decocker problem--for about $15 you can replace the safety detent and eliminate the decock function totally. It's just a safety at that point (Variant 3 they call it). Might be worth lookin' into as it's an easy fix!
Thanks for the heads up HK Dan
the one ragged hole becomes 2" in diameter, with very many largebore bullet hits. slowfire, 2 handed, no brace to steady you, full power combat ammo and a CCW gun? measure center to center, widest apart of 2 holes, 5 shots, 3" group is pretty fair, 2" is excellent, 1" is world class shooting. not that such slowfire stuff means anything for defense. It's more of a hunting sort of accuracy test. Such group shooting is far better done at 25 yds, and out there, 2" is excellent, indeed, many guns and full charge ammo can't deliver that, regardless of who is shooting.