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From what I can remember, the Colt M16 lower should directly fit onto most upper receivers, so long as they have the cut in them for the auto-sear. The older Colt AR (semi-auto) upper receivers didn't have the auto-sear cut, but I believe all of the A2 uppers and M4 uppers have the cut. The semi-auto Colt AR's had a different size pin/bolt hole to make it a little more difficult to directly bolt on an A2 upper. So, to put an A2 upper or other "full-auto" capable upper required an offset bolt. However, the M16 lower obviously doesn't have this problem.
So basically, unless your older uppers are from an older Colt AR (no relief cut for the auto-sear), you shouldn't have any problem.
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