Great writeup sqlbullet!
In my G20, I have found that measuring maximum case expansion around the pressure band has yielded VERY useful information about relative pressure. By pressure band, I am referring to the circumference of the case that is a little more than 1/8" above the extraction groove. Having done several dozen unique load workups for my G20, and I have found consistent measurements that track from one workup to the next.
I have found that maximum case expansion (Using STARLINE brass) consistently measures:
Starting charges: 0.431 - 0.432"
Book maxes: 0.433 - 0.434"
A bit beyond book maxes: 0.434"
Max acceptable expansion in my G20: 0.434"
Expansion beyond 0.434" begins to reveal Glock smiles (frowns)
Buffalo Bore and Underwood both produce ammo within SAAMI specs, but toward the upper end. They also conveniently use Starline brass. I almost exclusively use Starline brass, and it is pretty popular with 10mm handloaders. Great stuff.
Buffalo Bore loads consistently have a max case expansion of a hair hunder 0.434". That coincides with my observed upper limit in my G20.
When I do load workups, I carefully observe brass condition, and measure the expansion around the pressure band. As I approach 0.434", I know that I am getting to the maximums in my gun. I also know if I continue to push, I will get to Glock smiles, and I will have gone too far.
Examples of loads that expand to 0.434", or close to it, in my G20 chamber:
135 Nosler JHP @ 1700 fps
155 XTP @ 1425 fps
180 gr BB factory load @ 1335 fps
200 gr XTP @ 1200
Brass from my range ammo expands to about .0433". That would be a 180 @ 1175 fps.
An important consideration: different makes of brass will yield different expansion characteristics. Nickel is especially noticeable. That is one reason that I stick to Starline brass (it is also good quality and the best price). Also, that is why I do 100% of load workups in new brass. I do not use reloaded brass because work-hardened brass will expand differently than new brass.
Buffalo Bore or Underwood Ammo is a great place to start to establish an upper limit mark for those using G20 chambers. Each chamber is slightly unique, so my observations might not track with another G20 chamber. But for those reloading for a G20 barrel, most will likely find 0.434" will track consistently with max loads and/or Buffalo Bore and Underwood factory rounds. Pushing beyond that creates smiles, and loads producing smiles should be backed down.
I typically load 10 rounds per increment and increase by 0.2 or 0.3 grain increments depending upon the type of powder and how close to max. All are hand weighed and verified with check weights. At the range, I do the following routine:
Shoot one round. Holster.
Observe chrony reading.
Inspect overall condition (looking for smiles, brass flow, gas leakage at the primer pocket, primer condition, unburned powder, powder residue on case indicating poor chamber seal, etc. etc.)
Carefully measure the max expansion around the pressure band.
Chrony data, brass condition, and case expansion should track with expectations. If not, then I do not keep shooting the next charge increments. I have pulled plenty of bullets over the years.
If all is good-to-go, then rinse and repeat. For the next string, I use a different target location so that I can examine groups. This is a time-consuming process, but has enabled me to safely develop some nice ammunition.