As far as i know Oly arms is the only place to get a wssm upper they have workedall the bugs out of the operation. As said above the ejection port has to be larger along with some other internal dimensions the oly wssm upper is forged to those dimensions, also the bolt and carrier are made beefier as is the barrel, barrel nut ect. you can get an upper from oly for around six hundred and it is well worth the price, I am lovin mine more and more, I handload all my ammo so I can fit the ammmo to what I am hunting. Working up a combination mule deer antalope load now. thinking 90 grn SST @2800 FPS

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