Personally, I suspect that the undercut you mention was to prevent the carbon laden gas from getting back into the threads and jambing them up solid and jambing the muzzle brake on - as happens with Bren gas cylinders.

I let my son fire the Barratt when he was quite little. One of our Sergeants was alongside him in the fire trench. He winced a bit but went on to shoot five or so rounds.
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