Making repeated 100 yard shots on a USPSA target with a .45 caliber G21 is no big deal. It just takes practice.
Mackay nails it again--not just shots but respectable groups.
There's no reason that a current Glock pistol can't be as accurate as any other handgun, but you have to learn a new trigger technique and push through the travel in that trigger. That said, several things outside the pistol can make your life a living Hell of doubt and frustration until you figure them out:
1. Glock triggers smooth up with use. Google "25 cent trigger job," which gives the same result as cycling the Glock several thousand times. Do that trigger job on the stock parts and reinstall them in the gun.
2. Test ammo. A lot of it, especially FMJ training loads, will make the gun go bang but that's about all. Duty-grade JHPs tend to be very accurate. Chances are that your gun will shoot better with one load than it does with others. 10mm is expensive, so there's even more of a tendency to go cheap on ammo or components, especially with commercial reloaders.
3. Put the factory barrel back in and work with it for a while. Not all custom barrels are an improvement, especially if the drop-in ones. Truth be told, current Glock barrels are probably as good as any on the market. Some of the earlier ones had challenges, but that's far less common today.
4. Red-dot sights can take some getting used to. If you were ever able to shoot iron sights well, go back to those before you throw that pistol in the trash heap.
5. The Glock trigger stroke is far more like a DA revolver than like a fine rifle, so sink some finger on that trigger. With the striker down, determine where your finger needs to lie on the trigger to make the last millimeter or so of travel straight to the rear. Then start with your finger in that position. It may be a lot deeper into the trigger guard than you'd expect.
As Mackay pointed out, master that trigger stroke and don't let your skills die. You'll soon be winning lunch money from the unbelievers.
Okie John