What is the depth (length) of 2.1mm barrel jack on Arduino Uno R3? I'm thinking it is 5.5mm, but Radioshack also had 5.0mm depth. This port goes thru voltage regulator correct?
Neither of those dimensions have to do with the depth of the jack or plug. The 5.5mm is one standard outside diameter of a plug (inside diameter for the jack) and the 2.1mm is the diameter of the internal pin inside the jack.
There is also a common 5.5/2.3mm standard jack/plug assembly. The arduino uses the 5.5/2.1 design, but using a 2.3 plug in a 2.1 jack can make for bad/intermittent/no contact and trying a 2.1 plug with a 2.3 jack won't insert at all.