In my spare time I teach people to shoot. Been doing this for more years than I care to recall. The advice to look over the pistol from muzzle to backstrap is sound. That is the first thing you should do. Quality reloads aren't a problem; I've shot many thousands of them. Crappy reloads are the problem! Neither is lead a problem for a Glock barrel; SOFT lead is a problem!
If you've been shooting crappy soft lead reloads, or just plain soft lead bullets, then, you're barrel definitely needs to be scrubbed out with a bronze brush and solvent or paste. Personally, I don't think any of this applies to you because you don't say a word about either feeding problems or the pistol being, even slightly, out-of-battery while you're firing it.
I'll tell you what I think the problem really is; ready? I think the problem is with both you, yourself, AND the friends you've had test this G-19 for you. Please don't take offense; but, I've trained a lot of people - men and women - in how to handle and shoot pistols.
I got 'a tell you: When a pistol shooter stinks, he really stinks; and, often, his friends are no better. The rationale is always the same too. After making a mighty (and completely unskilled) effort, poor marksmanship or gun handling always becomes the gun's fault - Always!
Again, no offense, but you would have to have the crappiest Glock on the planet to post 6" groups at 5 yards. (What, the Hell, are you doing shooting a pistol at 5 yards?) Most of the people I hang around with don't even use the gun's sights at 5 yards! All of us simply point and shoot. Me? On a good day I'll put all of them into a 4" circle; and I'll do this just as fast as the pistol can go, 'Bang!'
You need some professional pistol training. My suggestion (for the 1,000th time) is for you to begin training with snap caps and regularly use, 'ball & dummy' drills every time you go shooting. Learn how to watch your front sight VERY carefully. The slightest wobble will tell you where that bullet would have gone.
When you load your magazines use some of your snap caps in with your live cartridges; and load your magazines with your eyes closed. Then mix the magazines up; and don't look at the magazine when you load it into the pistol. I think you'll be absolutely amazed at what the ball & dummy drills reveal. (Who would 'a thought - Huh!)
Hope this helps you out.