What is considered skilled shooting with a basic, issue, rack-grade (unmodified) service pistol? Let’s use a 9mm, 5″ barrel, open/iron sight, handgun, standing, two hands, slow fire at 25 yards (75 feet.) What should the grouping diameter look like?
First, what is the mechanical accuracy of the gun as fired from a machine rest? Most common rack-grade issue self-loading service pistols will be good for about three-inch groups at 25 yards. Some might be worse and others will do better but this is a reasonable guess. Tuned and accurized CMP Service competition pistols (“ball gun”) will shoot under three inches at 50 yards or better.
An old standard from practical shooting is to shoot a slow fire group freestyle (standing, two hands, no support) at 25 yards less than twice the size of the mechanical accuracy. So, with a factory gun capable of a three-inch group at 25 from a Ransom rest, a good goal is to shoot freestyle groups less than six inches at the same distance.
Skilled pistol shooters can do better. When preparing for military practical/combat matches (AFSAM, All Army, etc.) one of my training exercises is to shoot slow-fire five round groups (standing, two hands, unsupported) with an issue M9 (Beretta 92FS) on a blank target inside four inches. On a good day I can do this with all shots fired double action as well as single action. Of course, not every day is good. :) With a proper bullseye pistol, do the same thing with one hand.
The most important thing is that you see improvement each trip to the range. I recommend standard bullseye pistol targets (B-6 or B-8) at 25 yards, or the equivalent scaled target at a different distance, and shoot a few slow fire strings for score each range session. When your shots are mostly 9s and 10s, with nothing outside the 8-ring, you’re getting it!