Got an M4? Are your rear tabs broken? @#$%^!!!....
I'm going to rescue this receiver upper that I botched up. First, here's a CAD drawing, and a jpg for the CAD-deprived. Make sure you print it at 300dpi, and the pattern will be full-size. I made mine from .090" ABS sheet. Stock ones are .075" thick, but there is a bit of room for slightly thicker material.
http://www.air-sharp.com/M4%20repair%20tabs.dwg
The gun I'm going to resurrect with this repair is brand-new. I bought it so I could send the upper to Ka-Bar, who was very understanding about his showing up broken in shipment. (It's boxed, goes out tomorrow morning) so I'm going to repair this other upper that I have and make a whole gun out of the donor.
The upper you see little bit of was an early experiment. I had applied epoxy directly to the upper, and carved it to shape. Unfortunately, it made the assembly so stiff the tab snapped off the first time I went to snap it on. The lower assembly here is waiting for a strip and repaint, so I wasn't worried about it, and could focus on the repair.
But I digress. Your gun is busticated, and you aren't the first with the ailment, or the last.
First off, look for a replacement SRC upper. (Jungletoy used to have them, but now they have a new shopcart, and their search no longer brings one up). I paid $19.00 for one there earlier this year, so I know they are out there. Digressing again, but the point is, replace is better than repair. But let's suppose you live under an overpass, and you gun is broken. I don't know how you'd be reading this under a bridge, but I've seen the homeless with laptops...
I cut the parts out, and as you can deduce from the sequence of photos, one got welded in. The new part is located using the stock rear take-down pin, and the gun clamped shut after gluing. The squeeze bottle changes everything when using these glues. The included brush (fuzzy ball type) is only good for laminating large surfaces, (or use as an improvised torch when you're fifteen, and it's 1977, and you aren't properly supervised!)
The most important thing here is complete drying time. Oatey HD Grey adhesive (really, they should send me a x-mas card this year) will dry overnight, and hold the new part securely in place. I won't be standing on it, but the gun will stay together now.

I have more toys...than anybody...Google my name.