Side Charging AR question

[Rob]

I am helping my father with a side charging AR upper in 300 blackout.  I personally have only had and used the traditional T handle charging type.

The rear of the upper has a knurled  nut /screw with an oring seal on it.  It threads into the upper reciever where the charging handle would normally protrude.

One edge of the nut is flat ao that it will sit against the lower and won't spin out

Here is my question.  The nut is loose with a gap betwee. The oring and the upper, however if I thread it one more revolution in, it is super tight and the oring extrudeds.  What is the purpose of this nut/screw?  My assumption is that it is there to block gasses from blasting your face while firing.  Any possible solutions?  Was thinking of finding a thinner oring, or adding a washer.  Or removing it!

Will add a photo shortly

[Rob]

Photo.  The white washer is in place to close the gap (I added it to demonstrate the issue).  If i remive the washer and go one full revolution the oring extrudes.  Less and the nut/screw wont be in a position to allow the upper to snug against the lower and close.

[jay.sharkbait]

I’d chamfer the rec’r or take material off of the face of the knurled nut.

[Caseknife]

Take them both out and run the plug in to where the flat is on the bottom.  It is a bit loose, but doesn't cause any issues.  Or you could find a .010" shim or whatever it takes to snug it up.  Must be a BCA upper.

[bustedoldman]

Photo.  The white washer is in place to close the gap (I added it to demonstrate the issue).  If i remive the washer and go one full revolution the oring extrudes.  Less and the nut/screw wont be in a position to allow the upper to snug against the lower and close.

https://bearcreekarsenal.tawk.help/article/what-is-this-plug-on-the-back-of-my-side-charging-upper

Does this help?

[Dan-o]

tagging. 

Just picked up 2 side chargers for a son and I to build twin ARs.   

Ours also have that thing on the back that spins a little loose. 
Just got the final parts today, so haven't out them together yet. 

What is that mystery nut for? ? ?

Got ours from Bear creak arsenal. 

[Dan-o]

Ha!!!!

That was fast!!

Thanks

[Dan-o]

P.s. to the OP.......

Per the referenced link, don't shim/tighten it.
Its designed to operate loose.

[Rob]

Exactly what I needed!  Thanks everyone!