I have removed over a dozen barrel nuts all tight and not a single broken receiver. I clamp the receiver side ways in a vise with plastic sheets on the vise jaws to protect the paint finish.

Use a heat gun on the nut to expand it only and a good nut wrench.

There are no thread locker on these nuts. Just torqued. The black oxide finish on the nut is abrasive and has tremendous holding power once torqued.

Also removed quite a lot of nuts from receivers...

Anyone ever thought about leaving the mechbox and lower attached???

That is how I do it (with a thick rubber piece on both sides, in a vice), and nothing cracked ever... AND I did have to hammer a wrench to get it started... and it was a KA body..

I thought about doing that but I didn't want to take the chance of doing any damage to the lower. I think the real trick is what HKGhost showed, making sure you have sufficient clamping force just behind the barrel nut so least amount of flex happens. Not a bad idea though, as I can see little moving that way. I would rather not involve the lower if I didn't have to though.

I'm still amazed it takes the force it does to get these things off, King Arms really likes their parts tight.

Put a vertical grip.... on the top rail?

I've seen top rails torn right off the upper receiver by pressuring on the rail.