Tried that, actually - first with the allen wrench, then with an extension on the allen wrench, then with an impact driver. Thing's not budging, like not at all. The threaded end on the bolt handle starting to show some signs of shearing just above the point where it extends out of the bolt body, so I stopped and decided to check in here.

On my Model 16 .223, the equivalent bolt was very tight as well but nothing like this. Took standing on that to get it break free, but that approach bottomed out here.