Speaking as the devils advocate, you can do all the trials, extended trials, troop trials, pre production assessments, production evaluations, cost analysis, fault assessments, fault timings and whatever else you can think of over a few years.............. But guess what? There is absolutely nothing on this earth that will test a new rifle like full deployment to troops in the field.
But you have GOT to listen and act on the defect reports that filter back. Most of the faults that come back are generally trivial ones that can be easily put right with a bit of will and where withall. SA80 ejected cases tipping round. SIMPLE cure to remove .014" from the front/top edge of the ejection opening. Correct the shape to the cocking handle to make into a shaped deflector to boot and the whole problem is solved. The SA80 was but nothing compared with the Brens faults once it started seeing action in France. And look how that was revered - eventually.