thats also not really a bug: sometimes particles travel more than 1 px at the same time. that causes that they "jump" over any "Element-Wall" thats smaller than 2px.try using 2px of diamond. should work then
In fact, when I saw this I thought it wasn't a bug because there's a bit in Quantum Physics called Quantum Tunneling, which is the quantum mechanical phenomenon where a particle "passes through" some sort of barrier which has a higher energy than the particle, appearing to have no physical effects on the particle itself. I've found a picture from Wikipedia explaining it.
It hardly makes sense to me but scientists have debated that the concept is probable, not impossible, but never ever observed. It has been proven mathematically correct so it's safe to assume that it will work: It might be cool to have this theory implemented so that individual particles of unique characteristics such as neutrons are able to jump pixels and pass through some thin barriers.
Ugh, I know, if you feel the need to practice the art of necromancy, please use it on something other than thread. To the velocity gun... I can explain that for the sake of explaining it. I have a feeling it works several things, first of all there are two negative forces, and if multiplication is involved in their interaction then we get, for example, (-1)*(-1) = 1(Also due to the fact that it is pulling towards the edge, it is also pushing from the center outwards, therefore positive pressure). So we get positive pressure in the middle, so it's like squeezing a bar of soap, it tries to escape the pressure... doing so at speed.