What's That AR Thingy

[flabbydan]

(Sorry for the stupid thread title.)

I've often wondered: what's the purpose of the rectangular protrusion on the front left-hand side of the upper receiver, opposite the ejection port?

[bac0nfat]

I believe the corresponding cavity in the upper receiver is where the bolt cam pin resides when the bolt is in battery.

[ReconLdr]

I believe the corresponding cavity in the upper receiver is where the bolt cam pin resides when the bolt is in battery.

I was also thinking this. Without a picture it's hard to understand what the OP is talking about though.

[bac0nfat]

I was also thinking this. Without a picture it's hard to understand what the OP is talking about though.

I wasn't sure either, then I googled a picture of an AR15 upper and it was pretty obvious what he was talking about:

[flabbydan]

I believe the corresponding cavity in the upper receiver is where the bolt cam pin resides when the bolt is in battery.

That makes sense. Thank you guys for the replies. I feel so enlightened now.

[mythaeus]

According to reliable media sources, it's the compartment for the electronic components used to laser guide 30 rounds at the same time to hit multiple innocent bystanders.

Al

[bac0nfat]

According to reliable media sources, it's the compartment for the electronic components used to laser guide 30 rounds at the same time to hit multiple innocent bystanders.

Al

...and cook 'em at the same time!

[mythaeus]

...and cook 'em at the same time!

Only when you used baconator ammo.

Al

[ReconLdr]

Only when you used baconator ammo.

Al

And spray from the hip with the shoulder thingy popped up.

[cds_arms]

Answers are accurate. If you look at the slot in the top of upper where the cam pin rides you will see up front where it pushes the pin over and causes the bolt to lock/unlock when the gun cycles.

Pretty neat engineering by Stoner.