You might be dealing with two very short pieces of a bbl pin,,one put in from each side of the frame.
Sometimes, people change bbls and either forget, don't know enough, or can't get the bbl pin to go back into the frame.
They don't try and drill it clear. Maybe the replacement bbl didn't even have a clearance cut on it.
Whatever the case, the 'fix' is cosmetic. They cut two very short pieces of the bbl pin and round the ends a bit. Then punch one into place from each side of the frame into the hole.
It gives the effect of a bbl pin being in place. But in relity it's only the 2 short pieces, one on each side.
When you go and try to drive the 'pin' clear, you are driving one of those short pieces in and it stalls against the bbl threads.
About the farhtest it can be punched in is it gets flush with the frame/hole and that's it.
Usually, with a good frame wrench and bbl vise,,the assembly can be unscrewed with minimal damage to the bl threads as the pin pieces are very small and soft and only one side has been punched inward in the initial attemps to remove what appears to be a full length pin.
Might not be the issue, but it's something I've run accross.
Another is that people sometimes epoxy or superglue the pin into place when they are a loose fit upon assembly.
Also if the bbl and frame were hot blued together as an assembly, the hot blue salts can accumulate inside there and not get rinsed out. That can cause the inside area to rust quite aggressively and freeze the pin (and the bbl/frame threads) together.
Usually you will see some of the effects of the rusting or hot salts white 'bloom' around the bbl/frame joint if this is what is happening.
