Full Bore (or boar)?

Can't find out anywhere what this means, or its derivation. Does anyone know?
Thanks

I think it's 'bore' and relates to a river bore when it's in full flood. Near me in the UK is the river Severn. This has a very large tidal element which produces a large wave - surfers use it - it's called the Severn Bore.

It is indeed bore, but I'm pretty sure it's related to shotguns and firearms in general - you may have heard of the term "small bore rifles". Bore is the correct term for the calibre or gauge of a firearm. Shotguns in particular can be fitted with chokes that restrict the spread of the shot, so I imagine that "to go off full bore" is to do something whole-heartedly and without restriction.

Damnit! Beat me toomnit!! Shotguns can have full, three-quarter or half choke. No choke = full bore. Pellets discharged from a full bore shotgun embrace all and sundry exhuberantly, enthusiastically and indiscriminately.

In use, it indicates any system is operating at its maximum capacity. All restrictions have been removed.