First I will have to make an assumption. You are talking about a hinge(or break action) single shot shotgun?? The part you are referring to is usually not offered as a separate part , being a component of the barrel assembly. I have heard them called several things but forearm mounting stud is what I like. It should be secured and on a single shot gun such as described I would tighten in the dovetail and flow in low temp solder to solidify the joint. Peening and AcraGlas would likely work too but the solder is, in my opinion, preferable..
