AMSting~ Great questions.
Primers are a poor indicator of pressures. Flattened primers can be a symptom of excessive pressure - and care should be given to determine the cause of the flattened primers. They can just as easily be primers that weren't fully seated, or a headspace issue; thus giving a "false positive." Also, excessive pressures can be present without primers showing any symptoms whatsoever. This has been shown to be true in laboratory testing as well as the numerous photos of blown cases. Some primers have highly varying cup shapes and hardness so they do not consistently "track" from round to round. Winchester comes to mind.
One point to consider is that pressures will be higher in a "match" chamber than in a stock chamber. The reason is that the volume of aftermarket chambers are smaller. Same combustion event in a smaller volume = higher pressures. Granted the barrel itself will influence pressures too, but a chamber size that is smaller will have higher pressures. I personally would not shoot a round in a KKM/Barsto/Storm Lake/Lone Wolf barrel that I deemed to be unsafe in my Glock barrel.
My experience with the Gen 3 G20 barrel is fairly extensive. It is a solid barrel. I had originally intended to get an aftermarket barrel, but I found it to be totally unnecessary. I have personally done dozens of unique workups with at least 6 different 10mm powders and many different bullets. Bullet weights from 135 to 200 grains. These have been worked up to maxes (and beyond, in some cases). I have also shot the higher pressure "boutique" high performance ammo too with good results. I have less experience with .40 S&W , but my G23 barrel has been great too. No problems. At all.
What I have consistently found is that brass shot in Glock barrels do have a characteristic bit of a hog belly. That is normal. It is not unsafe. If brass has a smile, then that is a problem. It may be a symptom of excessive pressure for that system, or it might be an indication of premature unlocking of the slide. Either way, the cause must be explored.
When measuring the maximum expanded diameter, in my barrel and with Starline brass, the following measurements are consistently observed:
Starting charges: 0.431"-0.432"
Medium range loads are 0.432 - 0.433"
Max loads and a bit beyond will approach 0.434"
Beyond that and smiles will be observed.
Note that I have only encountered smiles with one workup out of dozens. It was a load published by Hornady, but exceeded the powder mfg data by a ways.
When all is said and done though, we cannot devine pressures from brass or primers. We can only follow pressure-tested load data and look for problems in our setups.
My suggestion is to keep your stock barrel, follow good load data, and look for indications of excessive pressures. I doubt you will have problems. What you will likely find is a highly reliable platform that is not particular about COL or bullet profile. If that doesn't work out for you, then an AM barrel might be the solution.
Last edited by Taterhead; 01-10-2013 at 22:00..