After helping build an RPK stock, the stock tangs on both Marui (and unfortunately, every other clone that was ever made) makes it very hard to make a nice solid of the correct diameters.
Cheaper stuff, while cheap, isn't guaranteed to last as long just because of the thinness of the material where the stock tangs are. Just look at a TM/Whatever stock and look at where it attatches to the stock tangs, and it's pretty darn thin material.
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