Did the prototype .276 Garand show any propensity for jamming, or dirt collection, during testing? Would the testing have been demanding enough to show up such a propensity?
Could have. Nothing in Hatcher's Notebook but then prototypes are expected to have problems.
The military acquisiton cycle is divided into phases. Broadly you could call them Concept stage, development stage, production stage, and fielding. DoD changes the names every new administration
https://dap.dau.mil/policy/Documents/Policy/Final back-120408_singles.pdf but if you can see there are a lot of activities from start to stop in any weapon procurement.
Big Corporation follow similar lifecycle models but these are not on the web.
Prototypes could be late in concept stage but I think these prototypes were early in the development stage. They would have been issued to troops for troop trials and more problems would have surfaced. Everything takes time and usage, problems fall out, and even then, at the end of it all, for example, the gas trap system of the Garand had to be changed even after the rifle was type classified.
Something that was not discovered in the Garand design till millions had been made and been issued world wide was seizing of the bolt lug in hot wet conditions. The roller bolt was developed in WWII but it once you have millions in the field it is too much of a mess to retrofit. The roller bolt made it to the M14.
Designs can always be improved. As was said "better is the enemy of good enough". At some point you have to get it out the door. All manufacturers hope to get a reasonably reliable product out there with as few problems as possible.
Development time is absolutely critical and is needed to surface as many problems as possible before the weapon hits the big time. An example of a weapon system that was not fully developed but was issued in the middle of a war was the M16. It had so many issues that troops died because of the unreliability of the thing. It was introduced far too early into combat. Now, sixty years later, it is a mechanically reliable item, but it still has fundamental issues that could not be changed because the retrofits would have required a complete redesign.
You can read the declassified “The report of the M16 rifle panel” There is a base document and at least ten appendices. You really have to search for Appendix 4, the one on ammunition. It is the most interesting to me from a lack of development view point.
http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA953110
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA953117&Location=U2&doc=GetTRDoc.pdf
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA953120
Once you get into these things you see all sorts of studies analyses, tests, that should have been conducted prior to the weapon entering service.
According the book "The Gun", it still took years of tweaking to perfect the basic AK47.