I've lived with wood heat for most of the past thirty years, and the very first fella I ever bought firewood from taught me to split even the biggest rounds using a single tool: A six pound maul.
The technique is called "slabbing", and it involves going around the edges of the round, whacking off chunks. You start by making a single edge stroke pointed towards the heart of the wood, then cross stroke that. A wedge will fall out. That first piece might take four or five strokes, but after that a single whack apiece will do the trick.
There's no limit to the diameter of the round this will work on- I've often gotten a whole week's worth of firewood out of a couple of big "unsplittable" rounds riddled with wedge marks and left behind by previous campers because they couldn't break through across the round.
Wedges and sledges are for splitting whole logs, in my experience. I've used them a time or two for fences and footbridges etc...but they're WAY too much work to use on a round of firewood.