http://dpharms.com/receiver-cross-members-p-78.html?osCsid=b47cb9c8e876fa7881e3fd34d10ca7cc
Receiver Cross Member.

I prefer the ak-builder ones...

I cut a dollar-store philips screwdriver shaft to length, and welded the bitch in there.

It was the only thing I had around that was .188ish-.250ish diameter, and I didn't care to keep. :D

what is the point of adding this "crossmember"?

Keeps the receiver from bowing

I make my own from 1/4 rod, center drilled and rivited in place

I take the AK builder cross members and cut them down slightly lower than flush on the operator side and weld them in. Then grind the weld down smooth flush with the receiver. Turns out peachy.

I use 1/4" G5 bolt shank.

Keeps the receiver from bowing

I make my own from 1/4 rod, center drilled and rivited in place

same but I weld. drill a 1/4" hole, slip in so its just below the surface on one side, rosette weld, flap wheel flush on the other side, make a divot with a 1/4" drill in the center of it and then weld that.