The other thread shows a burst a fair bit down the bore, is the fellow sure nothing was in there.

I still think, it may not be quite right to assume a .005" groove depth. I was probably seeing it wrong, but a fellow on the thread was demonstrating how to mic a bore slug. It looked like he had a picture of a 60* anvil. I think the right one for a three groove slug would be a 120* anvil on a calibrated micrometer.

It could be that a three groove barrel with wide lands like your drawing shows, could displace quite a bit more bullet metal and push up pressures some for the same groove diameter, maybe. I don't know though, that burst has the look like something was in there.