16 upper receiver insert
![16 upper receiver insert](https://i0.wp.com/rifleshooter.com/wp-content/uploads/2011/09/16-upper-receiver-insert.jpg?fit=980%2C653&ssl=1)
Place the upper receiver block insert into the upper. The ejection port cover will open when you do this. Once the insert is fully seated, close the ejection port cover.
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