I think most people would have a hard time shooting 3" groups at 25 yards with a Sig 210 let alone a glock!
Not to be argumentative, but most people (from what I've seen at the range over the last 20+ years) can't reliably hit a playing card at 7 yards, no matter how much time or what gun you give them.
That's exactly the point.
If you can't shoot to that level, then worrying about which bullet weight shoots best in the gun is a waste of time.
If the shooter can't approach the capability of the gun/ammo combination, then making the gun/ammo combination a little bit more accurate provides almost no practical benefit.
Here's an example.
Let's say that a shooter is capable (under absolutely perfect conditions--using a gun/ammo combination that is perfectly accurate--group sizes of zero from a bench) of shooting 8" groups at 25 yards.
With a gun/ammo combination that is capable of 4" groups at 25 yards, the overall group size (gun/ammo & shooter combination) will be, on average, 8.9".
With a gun/ammo combination that can do 2.0" groups, the overall group size will be 8.3", on average.
Making the gun/ammo combination TWICE as accurate--going from 4" to 2" groups only changed the overall group size by about 0.6", on average at 25 yards. That's an overall improvement of only about 7%.
Now let's change things and give the gun/ammo combination and the shooter comparable capability. So now, let's say the shooter can do (under perfect conditions) 4" groups at 25 yards.
With a gun/ammo combination that is capable of 4" groups at 25 yards, the overall group size (gun/ammo & shooter combination) will be, on average, 5.7".
With a gun/ammo combination that can do 2.0" groups, the overall group size will be 4.5" on average.
Now making the gun/ammo combination twice as accurate changes the group size by 1.2", on average--that's double the improvement we saw before. And if you look at it in terms of a percentage improvement, it's over 20%.
Here is the thing to keep in mind.
If the performance level of the shooter is not generally similar to the performance level of the gun/ammo combination, then the shooter error is dominating the equation and the gun/ammo combination are contributing much less to the overall group size than the shooter. In a situation like that, it doesn't make sense to concern one's self with incremental improvements in the gun or ammo in terms of accuracy.
But once the performance level of the shooter starts to get close to the capability of the gun/ammo combination, we see that they begin to contribute roughly equally to the group size and now it does make sense to start thinking about finding ammo that's a little more accurate.