Existed and can easily be made. The thing is that might be "illegal" even in USA and be pursued by agency having Bad Attitude Towards Freedom (infamous Bureau of ATF)

There were also ideas of guns concealed as zippo-like lighter,but such one probably that was not commercially available. Not complicated thing,but unfortunately it is probably one shot .22,and you might need some metal not a pen despite those thing almost look like pen. Simply spring knife longer than 3 inch might be better idea.

Existed and can easily be made. The thing is that might be "illegal" even in USA and be pursued by agency having Bad Attitude Towards Freedom (infamous Bureau of ATF)

There were also ideas of guns concealed as zippo-like lighter,but such one probably that was not commercially available. Not complicated thing,but unfortunately it is probably one shot LR22,and you might need some metal not a pen despite those thing almost look like pen. Simply spring knife longer than 3 inch might be better idea.