Knurled knob on end of AR upper ?

[kydave]

Does anyone know what this knob does? It has a flat surface at one point, which must be positioned corrected for the upper to close properly.

This is a side charging upper. This knob is where the charging T-handle would be on a traditional AR upper.

[Usmc0844spare]

https://youtu.be/dvopAxZAaEU?t=700

Why they have a screw to close off a hole when you would think they could just NOT BUILD IT WITH THE HOLE IN THE FIRST PLACE I do not know.

But, I have zero idea about side charging uppers so maybe you need that opening for whatever reason sometimes.

[Dougbert66]

Can't pull the BCG out of the rear of the upper unless the gas key has clearance all the way rearward. I assume the hardware seals that area where the normal rear charging handle would be.

[Barang]

^^^ was thinking that too. how about a photo of the rear view?

[sigstroker]

FM has that sort of setup but not quite as sophisticated. It doesn't really "seal" but keeps the charging handle slot closed up.

[ar15barrels]

That knob blocks gas from coming out of the upper and into your face while shooting.
A blowback action has lots of gas in the receiver since it's the gas that blows the bolt back and once the bolt is back, the gas can come out the back of the barrel.

[kydave]

Thanks. I found that info at the Bear Creek site. You said exactly what they said.

[KrizB]

I have a few side charging BCA uppers, like the others said, the screw blocks gas from coming out the back and stops the BCG when you take the charging handle off.
BCA makes three versions of each upper - rear charge, side charge, and dual charge.
They probably did it that way because its a cheap and simple solution to a problem, and it works.

Tip, don't overtighten the screw or else the o-ring will get squashed and not work.

[imarangemaster]

Originally Posted by Dougbert66

Can't pull the BCG out of the rear of the upper unless the gas key has clearance all the way rearward. I assume the hardware seals that area where the normal rear charging handle would be.

Correct

[edgerly779]

It keeps clueless owners from getting a facefull of gas by not inserting a CH. If you own one you need to knows how they function.