Very tight Mega Monolithic Upper ???

[StoneTower]

So essentially it has everything to do with the temperature. That post is exactly what im talking about. The barrel shrinks from cold goes into the hot expanded receiver when they are fitted the barrel swells and receiver shrinks hence crack. So yes temperature has everything to do with it cracking. Hence why i said use caution lol.

You can believe what you want but the receiver is not as elastic at room temperature as it is when it is heated. The molecules are farther apart when it is heated. When the upper shrinks it is essentially the same a driving a peg that is too large into a hole that is too small. If the hole cannot stretch enough to accommodate the peg, the material that the hole is drilled into will fracture. This is different than heating a bottle in a campfire and then dripping cold water on it. When the cold water hits the bottle, it cools only part of the bottle and the uneven cooling is what causes the bottle to fracture. When you use the heating and cooling technique to get to parts together as long as the larger part does not over stress the smaller part, you have a nice tight (almost permanent) fit. The problem is getting the two parts apart if you ever want to change the barrel. The cooling of the parts is even unlike the glass bottle example.

If you have an old barrel (with an old barrel extension attached), you could remove the index pin and turn the extension down slightly. You could then use jeweler's rough to lap the coating off the upper until you get a tight but proper fit.

Take care,

David