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On the current L frame Pro Series 686 revolvers, S&W is offering a 4" barrel with a 6 shot cylinder and a 5" barrel with a 7 shot cylinder (this one is also milled to allow the use of moon clips for reloads)

As I understand it, the 7 shot L frame revolvers have a bigger cylinder window that allows a larger diameter cylinder to be used.
Still, even using a larger diameter cylinder, which is needed to add the extra round, I`m thinking there isn`t as much metal between each charging hole in a 7 shot cylinder as there is in a 6 shot cylinder.
Is this correct?

I know that modern metals and heat treatment are capable of withstanding pressures that older, less sophisticated metal and heat treating techniques couldn`t handle.

The seven shot L-frame uses the same size cylinder and frame window that the six shot uses. The holes are just drilled closer and the timing is adjusted. You can convert a six to a seven, and vice verse.

The seven round cylinder places the timing notch between the chambers, in the thickest part of the cylinder. The six shot places the notch right above the chambers at the thinnest spot. Supposedly, because this is the most likely spot for cylinder failure Smith and Wesson rates the seven shot cylinder as the stronger of the two.

I have two 686s, one a seven shot and one a six. They have each fired well over a thousand rounds of .357 with zero problems. I would prefer the seven shot over the six except for the fact that my favorite speed loader - Safariland - doesn't come in a seven round version.

You are correct, in the 79 years that S&W has been producing 357 Magnum revolvers, they have learned a thing or two about manufacturing and metallurgy that allows smaller, lighter and stronger revolvers to exist today.

That is why we now have 12 ounce 357 Magnums that can be carried in your pocket all day as well as 7 and 8 shot 357 Magnum revolvers for belt carry.

None of this is new to the market. These are time tested and proven designs. The 7 shot L-frame revolvers have been in production for just under 2 decades. If there were a strength issue that S&W had missed accounting for, it would have showed up by now.

The 8 shot 357 Magnum revolvers have been out for 18 years now.

Buy your revolver and enjoy it