Other than the bolt, the carrier really can't fail as long as the key is properly staked.

So a replacement bolt is handy to have, along with a set of gas rings and a bolt repair kit

If you have to break down the lower for any reason, it's not hard to lose a spring or pin.

Here is a link to a few different kits, this will give you an idea about what to search for.

Lower Parts Repair Kits

Unless you have a sqib, there really isn't any reason to have a replacement upper or barrel. Wait until things relax and prices and availability improve to buy those big ticket items.

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