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Are all upper receivers interchangable with a "standard" lower receiver ? ie; a .223 lower convert to a 9MM

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Yes, AR15 type receivers in different calibers are interchangeable with the .223 lower receiver like the .450 Bushmaster, 6.8 SPC etc., as long as the action is the same length. In addition to a 9mm upper a 9mm mag well block must be used.
A 7.62 X 39 Russian conversion will fit the .223 lower receiver but a 7.62 X 51 Winchester .308 action is too long to fit a .223 lower receiver. 
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