I've managed to strip the head of the grub screw which fits inside the barrel lock plunger.

The grub screw had a much shallower flathead slot than is featured on the typical screw, roughly around 1/6th as deep. I wonder if that's an anti-tamper feature? It's an AUG-Z which is a Euro equivalent to the USR, with some differences. Luckily have a complete rebuild kit for the QR mechanism, so all the replacements parts are on-hand, I just need the existing plunger out. With the head of the grub screw stripped, are there any options other than drilling the screw out?

Thanks.

You're going to have to drill through the pin that retains the plunger. The only way to do that is straight through the grub screw. Once you kill the grub screw, your pin will drop out, but likely will need to be pulled out with pliers. The barrel release cavity will have a bit of metal shavings that will need to be cleaned out.

A couple of us have already done what you're about to do. It's not too hard to do, just take it slowly...

Thank you. I got it removed without too much trouble.