I think there are some typos in the assignments in the computation of hashCode().

https://github.com/aidancbrady/Mekanism/blob/master/src/main/java/mekanism/common/multiblock/SynchronizedData.java#L41

What makes you say that? Everything looks and compiles in order for me.

Fixed!

I may be wrong but shouldn't it be "code +=" instead of "code ="? It seems that the whole method returns 31 * volume.

Ah, you're right.