Excellent work !
If I understand you correctly, the throats are 0.500", the groove is 0.501", the bore is 0.492", and the tapered cone in front of the case is 0.280" long.
I've heard different measurements from other 500 S&W owners. It's possible that there is some variation from gun to gun (ya think ?) or it is possible that there is some measurement error. Either way, at least we have some ballpark figures to work with.
What I have consistently heard, is that the throats are tight, 0.500" or 0.499", depending on the source. That seems typical for current production Smiths.
I've also consistently heard that the tapered cone in front of the case is long, though your 0.280" figure is the longest to date.
You didn't report the chamber ID at the case mouth, but I've heard that's pretty tight, not leaving much clearance for rounds loaded with oversize cast bullets. Most people size to 0.500", rather than 0.501", for that reason alone.
As for why Smith does things the way they do, you'll get a headache trying to figure that out.
It's possible to ream or lap the throats to 0.501", but ...... if it shoots OK, I wouldn't worry about it. As they say, don't fix it if it ain't broke. Most people say the 500 shoots well with a gas check. I've heard mixed reports on plain base bullets.
To submit a change, a screen shot would be ideal. You can either email it to mtngun at gmail, or you can post it on this thread if you know how to do that.