This is a part of a message chain first posted in this area, but probably better found in the Home Gunsmithing area -- and I moved it there. But, double pedro's comments, below -- a part of that message chain -- were too informative and useful NOT to leave here in the FAQ area, as a separate topic.
His name isn't attached, and it should be, bu I couldn't do that for him. It's his work, though, and well worth the read.
Gary - Your insights and concerns are on target (pardon the pun). At what level a particular issue affects accuracy is hard to quantify unless you have a very reliable and repeatable way to shoot groups like a Ransom rest or carefully shooting from a sandbag (even that may not be good enough without a great deal of skill, concentration and patience). I've fit four barrels in my SP01s (two factory and two aftermarket). Carefully analyzing the problem, I came up with the following approach. Whether this is "worth it" is entirely up to the shooter.
1) Fix the lateral play of the barrel in the slide. This requires truing the inner sides of the slide so that they are parallel or become slightly wider as you move towards the muzzle. Once this is done, then the width of the barrel must be increased (by welding thin shims on either side) so the barrel lies along the axis of the slide and the bottom lug is perpendicular to the mid-plane of the slide.
2) Fit the top lugs. The barrel should contact the slide at the top lug near the barrel hood. The contact should be made across the lug as much as possible. You can use modeling clay and paper "feelers" to see how close the fit is in various spots. Chances are, the barrel may contact someplace on the barrel hood, which is less than ideal. Fitting is done by careful filing and possibly adding metal (by spot welding) and more filing. This can be a long, tedious trial and error exercise using High Spot Blue (Dykem).
3) Re-cut the lower lug. Once the side to side play is eliminated and the top lugs are fit, the last step is to re-cut the lower lug. You will probably have to first add metal to the lower lug so there is something to cut to fit. This requires spot welding inside the kidney-shaped cut-out to add metal where you need it. Then, you have to clean things up with a file and use a special cutter to re-cut the hole. The cutter I used is available from Brownell's for 1911's but needs to have the shaft diameter reduced so it can be inserted far enough for CZ's. This cutter is used in place of the slide stop and is turned by hand to cut the lower lug in place (i.e. the slide and barrel are mounted on the frame and pushed into battery as the cutter is rotated). This ensures that the lower lug is cut so that when the slide stop is inserted, it will (just) keep the barrel locked in the slide and maintain that contact for a short distance during initial slide travel (while the bullet is still in the barrrel). The way to check this is to push your slide about 1/8" to 3/16" out of battery and see if the barrel rattles or otherwise moves at the muzzle or chamber. If it doesn't (and it cycles smoothly), you should be good to go.
One last point, I don't know whether any particular CZ models are fit better than others. However, I recently purchased a Shadow and found the barrel on it was very well fit. I'm still breaking it in and haven't carefully benched it but expect it should group better than average. I have no idea whether this is indicative of all Shadows. For any large production service pistol, I think it is unreasonable to expect anything better than what you get with your average CZ - accurate and reliable.