Circuits have replaced Nodes in this game. However, you still find them in the exact same looking containers that Nodes were found in in Dead Space 2. The Circuits come in all kinds of different varieties. You have the 4 basics, which are Reload Speed, Rate of Fire, Damage, and Clip Size. Unlike Dead Space 2, where there was a set amount of these things for every weapon, in DS3, if you want all damage and nothing else, go right ahead. They also have combined Circuits. These take the attributes of two circuits and combine them into one nice package. These are your best bet for upgrading weapons.
For the achievement though, simply take any ol' circuit and slap it on any weapon, and it is yours.