Key Points: The MRL 4–6 transition is where programs fail: designs that passed lab validation routinely break down when the process, material, and volume constraints of real production are applied. DFM must happen before MRL 5: design for manufacturability reviews that arrive at MRL 5 or later cost significantly more to implement than those built into MRL 4. Tolerance stacks compound across assemblies: what holds at ±0.25 mm (±0.010") on a single prototype part can create interference fits and seal failures at production volumes. Material qualification is not material selection: choosing a material and qualifying it for your process are separate activities with separate timelines — conflating them causes schedule slips. Vertical integration compresses transition timelines: a manufacturing partner who controls machining, converting, dispensing, and inspection under one roof removes the coordination gaps that slow MRL progression. Why MRL 4–6 Is the Hardest Stretch in the Program Most programs expect the hard engineering to happen at MRL 1 through 3 — the technology assessment, the feasibility modeling, the early lab builds. By MRL 4, the prototype exists. It works. The team has validated the design concept in a lab environment. The assumption is that the hard part is behind them.