If you fully upgrade your weapons you get machine parts AND shards when you sell them.

[Vault-TecPhantom]

Thought this was worth sharing as some will undoubtedly miss this. Don't sleep on upgrading your lower grade weapons just because you're inevitably going to get blues and purples later. If you take the time to upgrade lower gear while you wait for merchants to stock higher tier weapons you can sell them and get machine parts which can be used to upgrade those higher tier weapons later. Saves the time on hunting them down while also not having to sell resources to contract givers.