Quote History Quoted:
It is not an oil hole. It is used to retain the front take down pin detent during assembly. This is from 2003 Colt Armor course notes.
18Z50
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Sorry, but that just isn't true.
If you were instructed that, the instructor was in error...
While it's nearly become an "AR-15 Urban Legend" the hole on the fence can't retain anything except the tail of the detent spring.
If you look at the below pic, it shows that the tail of the spring only goes past the hole by ~0.18" (The bottom of the hole is the right edge of the green tape).
How would this retain the detent? It can't and it doesn't.
Here's a pic of the "Pivot Pin Detent Installation" section from the
Colt M16A2 Rifles and Carbines Armorers/Depot Maintenance and Repair Manual, Second Edition 1991, Copyright date 2004.
This is the technique that I recall Colt teaching in that era, and I don't imagine it has changed.
For anyone who might care, here's the removal instructions-
Again, no mention of the hole on the fence in either the assembly or disassembly instructions.
The hole is for maintenance of the spring and detent.
It allows any water to escape, and it allows you to keep it lubricated and moving freely.
It serves no purpose in assembly or disassembly.