That is the plug that takes the place of the charging handle on a side charging upper. It stops the bolt from falling out of the upper when you open up the rifle and plugs the hole in the back of the receiver to prevent hot gasses from blowing into your eye in the event of a ruptured case or pierced primers. You could have gotten a face full of hot gases from those primer failures that you just experienced if it was not installed.
The operating handle on the side charging upper is held on to the rifle bolt by an Allen bolt that has to be removed along with that plug to get the bolt out for cleaning and maintenance. The flat part has to face down (toward lower receiver) when the plug is installed to allow the upper receiver to close all the way.
The operating handle on the side charging upper is held on to the rifle bolt by an Allen bolt that has to be removed along with that plug to get the bolt out for cleaning and maintenance. The flat part has to face down (toward lower receiver) when the plug is installed to allow the upper receiver to close all the way.
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