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1. What year did Colt discontinue the "double screw" front takedown pin (or variations thereof), and go to the standard, proper one?
About 1999 but some lowers were drilled for a pushpin before that.
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2. What year did Colt start installing the proper M16 bolt carriers in their AR's and not SP1 carriers with unshrouded firing pins and cut down rear shrouds?
Around 2007
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3. I'm assuming that all modern Colts have the proper M16 bolt carrier and there isn't some oddball 5.56 model still being produced with nonstandard bolt carriers?
All of them should have a regular M16 carrier
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4. What year did Colt start using standard sized FCG pins?
About 2009
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5. What year did Colt stop installing a separate auto sear block and went to simply not machining out the "web" in the lower receiver where the auto sear would have gone?
About 1998
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6. Are there any parts still in a Colt AR that are there to make it a legal semi-auto, but might cause some sort of reliability problem due to it not being the actual military part?
A notched semi auto type 2 hammer intended to be used with an exposed firing pin semi carrier. Colt still puts this crap in the lower. It can be replaced with a type 1 semi auto hammer from Specialized Armament.