If someone lacking any authority, other than that they bestowed upon themselves, grabs my wrist (assault), or tries to take my gun, there will be a struggle. I am not going to assume that I need to surrender my weapon to this person, who I do not know, who is not a police officer, and whose intentions I know nothing about,.
I can be fairly confident, however, that struggle would not be taking place in a bar.
This is not the same situation as a police officer taking custody of a weapon during an investigation, but one non-LEO taking it upon himself to use force to disarm another non-LEO. Said force is only justified if it can be shown that the armed individual is acting in a threatening manner, as private citizens cannot engage in preemptive self defense. This sounds somewhat similar to that recent case where one Walmart shopper took it upon himself to disarm another (properly licensed) Walmart shopper in Florida.