Funny, my Lee 2nd Edition says in their 40 S&W section "Do not use reloads in Glocks or similar guns with chambers that do not fully support the cartridge due to the intrusion of the feed ramp." I don't believe I'd use Lee's data if I was reloading for a 40 S&W Glock, but maybe that's just me.
Well, two things.
Handloaders are prone to "push" their loads.
And ... handloaders (especially new ones) are prone to being idiots who don't pay attention to the rules, particularly neck tension, proper load development, and seating depth.
One thing you ABSOLUTELY have to avoid on the little 40 S&W is bullet setback, or deep seating. A "moderate" load can go to "critical mass" in just a few hundredths of an inch on 40 S&W, ramping pressures up from ~40k to ~70K+.
Shooting ammunition out of an unsupported chamber is something I do just about every weekend. Have done so for 15 years, shooting reloads, and still have all my fingers.
Proper load development on an unsupported chamber should be:
1. Load 5 rounds each, from the
starting load on up, with .2gr increments.
2. Each batch of expended casings, bag in a ziplock and label (so you can examine under good light, later)
3. Watch and FEEL for "guppy belly" as you progress. This is a sign that the pressures are starting to exceed the web strength.
4. STOP when the guppy bellies are easily detectable on 4/5 or all of ejected cartridges. (3/5 or more if shooting mixed brass)
5. Pull down remaining cartridges.
Also, try to keep headstamps THE SAME. If you are shooting mixed brass, SORT it by headstamp. Some brass is naturally harder than others, or have walls / webs of differing thickness.
Glocks have an unsupported chamber that exceeds the webbing area. You'll notice guppy bellies starting with a circular line .230" or so from the base; the thick web extends only .180" on most brands of brass. Thus the guppy belly will be a bulge that is more pronounced towards the front edge.
Pay CLOSE attention to your seating depth and tension. Load dummy rounds with that brand of brass (no powder or primers) and test chambering 2-3x each, measuring before and after, by releasing the slide from full-rear (do not guide forward). This will give you an idea of how much setback you will experience. If the setback is .010" or greater, that's bad, on a high-pressure 40S&W. You will almost always see SOME setback on repeated chamberings but it should only be .002 to .005".
The reason we test load 5x each level is because not every piece of brass is the same, not every powder meter is the same, and not every bullet will set back the same on chambering. Shooting a string of 5 each on load development will give you the minimum amount I'd consider using for load development. (I usually do 5 for each gun I own in that caliber).
The reason to bag the brass and label it separately after firing is so you can examine it closer back at home, with magnification and good light. You might see stuff you miss when casually observing it at the range.
That's my advice.