Probably the best short and sweet definition of what a gas ring does is summed up in Mike Wood's excellent L frame article over at the
Revolver Guy blog. "The gas ring’s purpose is to prevent carbon and lead from building up between the axle (“arbor”) and the cylinder itself, which would impede the rotation of the cylinder." A gas ring is even more important on a blackpowder revolver, however it is still an important part on a modern revolver. There are obviously variables at play when it comes to encountering issues on these guns (type of power used, how often the cylinder is removed for cleaning, b/c gap, etc...)
The K frame magnums have a long history of gas ring related issues. Extreme heat and recoil from firing magnum rounds caused the cylinder mounted gas ring to move and impede cylinder rotation. Massad Ayoob stated in the Sept./Oct. 1980 issue of American Handgunner "The Combat Magnum is theoretically ideal for use in law enforcement. The only problem is that, especially in the stainless version, it is not unknown for the gas ring to move foward and bind the gun due to buffering from magnum recoil." S&W knew this and actually made a design revision in 1972 to all k frames (not just the magnums). They moved the gas ring from the cylinder to the yoke. This opened a whole new can of worms and infuriated police officers and competition shooters (a lot of crossover there too). Guns with the yoke mounted gas ring allowed carbon to build up much faster on the cylinder and impede function. There were so many complaints that S&W went back to a traditional cylinder mounted gas ring in 1977. If you look at a 67 no dash and a 67-1 side by side you will notice this.
With the new model 19, 66-8 and model 69 there are no gas rings at all. This will make all of those models more prone to sluggish cylinder rotation than other S&W revolvers. The K frame magnum is always going to be a rob Peter to pay Paul type of revolver. The deletion of the gas ring was unnecessary on the model 69. S&W could have done what Taurus did on their mid size .44 Magnums, and just went with a slightly shorter cylinder.
All of this is easily avoided with the tragically overlooked half lug/lighter barrelled L frame revolvers. The 619/620 is within an ounce of a model 19/66 and the 686 Mountain Gun weighs even less. These also allow the use of a seven round cylinder with offset cylinder notches. It really makes no sense to bring back the K frame magnums when this can be done. The lighter barrelled S&W L frames might just be the most underrated revolvers of all time. Sadly, poor timing, poor marketing, concerns over the two piece barrel/lock, and new model numbers caused the 619/620 to be stillborn in 2005. The 686 Mountain Gun was never a long term standard production item. With the revolver's comeback in popularity, I think it would be a good idea to give the lighter barrel .357 L frames another chance.
Speaking of black powder, the S&W breaktop replicas (Model 3, New Model 3, Schofield, etc...) suffer from this as well. Instead of slightly stretching the frame to allow for a longer cartridge, Italian makers simply eliminated the gas ring for more clearance. This is one reason why these revolvers are not nearly as popular as a SAA in SASS shooting.
I am not dogging the new 66-8, 19, or 69. I have read great reports from folks, including those on this forum. This is not something that will bother everyone. However, it does open a new can of worms and would be avoidable altogether with a half lug L frame. Hopefully someday folks will re-discover the lighter barrel .357 L frame and it will make a comeback.
Here are some great links and the article referenced above:
https://revolverguy.com/the-smith-wesson-l-frame-story/
http://smith-wessonforum.com/s-w-rev...new-model.html
http://smith-wessonforum.com/s-w-rev...-cylinder.html