This may have been covered already, but...
Has anyone had luck removing the "block" from the Colt receivers? Looks like you could drill and tap it and then insert a fastener of some sort and pull it out!!! Or, of course, it could be milled out, but if the pin can be removed without too much trouble, I could avoid the effort of milling it out.
  I am getting a registered drop in auto sear and would like to use my Colt as the host gun.

just buy a bushmaster lower(less headache). Not to mention if you F*ck it up you are out a lower receiver.
just my 2 cents.
matt

Maybe this will help:

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