Do you reload? Have you ever worked on the bolt? Which "pin" are we talking about?

I don't know of any pin that can easily "back out". If the BAS backs out then it is a simple fix. Tighten it securely. The bolt head cross pin cant move due to the firing pin holding it in. The possibility of the firing pin spring releasing tension is not probable unless parts were missing from the factory, and remains unlikely that it would back off.