It's the word key that throws me here. I'm thinking key as in key stone, absorbing pressure from two sides co pressure, com-pression. It could be that I'm not familiar enough with your timber framing jargon in this case.
As for splitting, I don't think it's an issue of the peg, round or not. It is another matter altogether, could be in the area of accuracy and appropriateness of the joinery, something like that. Take the simple example of hammering a nail into a board. If I hammer a relatively small nail close to the end of a far thicker board of say spruce it's likely to split that board. At the same time, if I clamp a thin strip of lattice, of the same wood, in a vice, (I could just as well pre-drill for the nail. Or don't forget blunting the tip, that one works wonders, or even lubricating the nail, sometimes I use that one too), I can hammer a big nail just as close to the end take it from the vice and there will be no weakening of the wood relative to the nail. If I split the wood then the split is just as likely to go around the nail/hole. In other words it's the wood structure influencing the split not the nail or peg in our case.
I think there is a notion of the mechanics of the joinery and the structure and properties of the wood going on here that may be intuitive but is incorrect.
Another thing is the matter of which looks better, round-like or square-ish. And to add to that, in my examples up there the openings for the pegs are off-set, so some drawn effect taking place on those. I was surprised that there was no splitting with them.
Greetings,
Don Wagstaff
