Outsourcing is primarily an economic decision: is it cheaper to buy or to make? Only secondarily, in most cases, is it a matter of obtaining a better part. One exception is the magazine, which can make or break a gun's success. It has to be made right. As it happens, Mec-Gar has the talent and equipment to make a better magazine than Walther ever could, and less expensive besides. The same is true of small investment-cast or MIM parts.
The Ranger-made Interarms guns all used triggers, hammers, safeties, trigger guards, hammer blocks, ejectors, extractors and rear sights from Hitchener. These required no machining,* only cosmetic polishing. Pins, springs and grips are always outsourced. That leaves basically just the barrel, slide and frame to be made or machined in-house.
This means that the finished product relies heavily on scrupulous, in-depth quality control and final inspection, as well as extensive testing after any change in design, materials or processes. In this regard some companies do much better than others; it depends a lot on managerial philosophy.
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