So the testing shall continue.
I have shot enough for some conclusions IMO though.
TLDR: These extensions can work with caveats. You can absolutely shoot 50 problem free rounds with these. That being said it hasn't been my experience that this is what you should expect reliably, downloading to 40 appears to help.
The main negatives from my analysis is too loose tolerances on the print believe to be rounds hanging up on the transition at the extension during feeding. This is allowing too much play, at first this didn't appear to be an issue, however I had a few instances of what I believe is the rim of round catching causing the rounds to stop feeding.
Tapping the mag would get it to feed, eventually the mag would start feeding correctly.
I'll do more testing though it looks like downloading the mag to 40 rounds may help... the idea here is that the rounds I believe all stay inside of the mag and prevents this issue. At least once I got down to about 40 rounds left on a mag two separate times the mag started to feed and the rest of the mag feed without issue. The last mag I loaded to 40 and shot w/o issue and then had to call it a day.
The second thing going against it is the weaker lowest pushing force of the spring... there has to be a technical term for this.. anyhow while this makes the mag work IMO... this also makes the mag more sensitive to dirt and debris. That being said I can't attribute failures I've had to the 'weaker' spring. I only expect to have to clean the mag earlier than an OEM mag; still regular maintenance with the CMR by my account.
As an aside on my reported first mag jam I thought I'd show what a mess ~120 rounds with the last ~30 rounds being that Armscore dump I had mentioned looks like. If you look at the image of the bolt below you will see two balls of carbon, powder, and oil that where scraped off the rails... if I had to guess I got unlucky and one of these balls got into the mag. I've shot this a lot, done plenty of mag dumps / heavy shooting having a giggle... this is by far the most carbon choked I've seen it lol even shooting 400+ suppressed rounds of CCI. I’m sure my suppressor is a hot mess.
Anyhow here are the pics for perspective:
I will continue to play around and work on these, I'm determined to get a reliable box of ammo fed through this gun per mag now lol. I've a few things I want to try out still as I believe it is possible with a little TLC.
I will also note on the mag that broke, it appears to have been printed along a different axis allowing a short z-axis across the extension's lip... This could have been a factor in what otherwise appears to be a well constructed part as vertical layers typically don’t bond as well and represent the week axis on printed parts. I'm not even 100% sure I'm interpreting what I'm looking at correctly having not much experience with SLS printing to say the least lol. But believe this may have been a factor. Another difference is the post processing or mayhaps different printing parameters... I can only guess. I speculate that one of my mags was a prototype / early production example... this is based off the z-axis delta (this could be a print space fill / cost engineering factor though), look, texture, and the retention tab included... on the broken one, the retention tab was painted, shown some wear, and was too short, to the point you could 'loose' the tab in it's slot, and it was very difficult to remove as the paint appeared to bind with some loose Nylon powder and had to be 'broken' out... anyhow after I pushed too far I had to back out the retention nut and 'scrape' it out... it isn't even long enough that when placed to hold the mag the nut lands on the edge of the tab. While I’m not sure how much of this (other than the tab) is just print and process variations.
Regardless of what the actual reasons behind the differences between them, these deltas give me the impression of possible revisions and improvements between the two extensions.
I sent Will at S&T an inquiry just before the holidays and haven't heard back, I'll include some of these new details. I'll post back anything relevant.
I've included some photos below so you can see the differences in the mags.
Retention tab difference:
extension difference - enlarge to see z-axis lines (ignoring drop damage):
Lastly the included springs appear to rust a bit easier than OEM. Mostly I mention this to recommend a light coat of oil before installation just as I do on the OEM springs, they are both going rust unprotected.
Below is new OEM, new S&T, after ~80 rounds inspected 4 days later: