(I think) you're partially right . . .
The S&W barrel extension would most likely be used if you kept the S&W bolt and bolt carrier. The S&W bolt is mated with the S&W barrel extension to provide the proper head spacing.
That's what Black Hole Weaponry was attempting.
I spoke with someone from BHW, who thinks that the DPMS bolt carrier group when used with a DPMS patterned barrel would be a drop in replacement.
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