I’m no gunsmith but I had an old Winchester (1907) that had a similar section of chamber fall off.
On mine it was further down the cartridge wall (straight walled case) and after firing prevented extraction.
In my case I believe it was from a flaw in the steel; no way was it made from something someone did as you could see the natural surface of the steel.
So the question in my case was how safe is the barrel and was the imperfections limited to what has already fallen off.
I had no way to insure this but it was a low pressure round so I did a bubba repair with high pressure epoxy, held it at distance, muttered “well here goes nothing “ and touched one off with nobody around…
I absolutely do not recommend this, I cannot stress that enough.
End of the day god protects fools so it worked fine and I put about 50 rounds or so through it with no issues until I sold it with the repair still holding as a wall hanger to a guy who parts them out.
At 303 pressures I personally wouldn’t shoot it, and if I sent it for evaluation to a gunsmith I’d do so with the intent to have it returned repaired which would likely be a barrel swap.
Unless you know exactly what happened to create that pocket I would err on the side of caution as you don’t know if the barrel integrity is stable, or at what point it becomes unstable.
Likely not a thing but could be and if so, would it have been worth it?
