Given a M4gery, what spare parts, given normal wear and tear, should I keep on hand? I am thinking about operator level repairs/part replacement and parts that are likely to wear out and fail under normal (range) usage. So no armorer's work like barrel replacement or any thing that would require special tools. By way of example something like spare gas rings. I see a lot of lists but they seem to list parts that the average use would never replace and/or unlikely to break. So what I am asking is what is a realistic list.