You may have used too small a piece of PVC.
The barrel tapers about there and I'll bet you ran into that and not the handguard as it is the same size inside all the way to the nut.
A better idea is to get one of the two wrenches Shooboy makes and a set of the vise jaws made by Tacticool.
You can find both of them listed in the classified section at the top of the this page.
They will allow you to take the barrel nut off much easier and with little or no chance of damaging the receiver or handguard.