One of the downsides of machining a cast metal part is the potential for voids. While I’ve only encountered this a few times, here is a great example on a Remington 700 bolt that was threaded for a Badger Ordnance over sized bolt handle. Note that the void wasn’t exposed when the thread was cut on the lathe, but when the handle was in the blast cabinet.
To work around problems like this, some smiths will replace the handle with a machined part or thread an undersized screw through the center of the bolt handle.
I wanted to see when this would fail, so I decided to use the bolt as is. Surprisingly, after a few hundred rounds it hasn’t broke yet. If and when it breaks, I’ll install a TAC OPS bolt handle. See how here.