The 357 Maximum seems an excellent case for a light rifle, big enough for fun with black powder but not so large that loading down with smokeless is any problem. I wonder, though, how long they'll continue to make it.

So, how far out might one seat a bullet in the 357 Magnum case, perhaps just the bottom band of a lead bullet. I think I read folks have loaded bore rider bullets so, and I wonder how well it works.

I depends on a lot of things. But mostly how are you going to use the cartridge.

You need enough bullet in the case to keep it straight and from moving. 

A tube fed magazine will need a lot more hold on the bullet than a single shot where people can and will load it outside the case.

A revolver  tends to need a decent crimp so the other bullets in the cylinder don’t fall out under recoil.

In the example you gave, depending on how the bullet fit the chamber and bore, you have to consider if the cartridge becomes a press fit, bullet into the lands, is there enough hold on the bullet to keep it from pushing back in the case, or will It do it the same every time. 

You also need to be able to extract An unfired round without leaving the bullet behind and dumping all the powder into the action.

Once All those issues are worked out, then you might want to fool around and see the effect a few thousandths difference in neck diameter might have on accuracy. 

Just remember to not have such a tight fit that it deforms the base of the bullet when you seat it.

I tried this very thing in a .357 Maxi chamber. In a word it didn't work too well. It's a heckuva long jump for the bullet out of a .357 Magnum to make, even if seated way out. Accuracy was "minute of deer" with multiple loads, but nothing close to what the Maximum cartridges deliver with 200 grain bullets.

Hmmmm.  Consider a gas-checked mold design with no gas-check.  Using the rebated base as the only part that goes into the case.  Consistent minimal seating depth.

Well, as a GENERAL rule for broad range calibers, I've always considered 25% of the length of the bullet must be seated in the case.  I recognize that there may be a lot of exceptions to that rule, but it works in most situations.

Here's a 40-50 SS with the bullet seated just on the baseband. This is my ram load.  The bullet is a Ron Long tapered spitzer over 55 gr Swiss 1-1/2.   

Bullets can be seated any way you like. One of our shooters seats bullets deep into his 45-90 cases for chickens. The tips of the bullets barely stick out of the necks.