Hi and welcome. you will enjoy your 686. The 357 125 gr "full house" is availiable from Federal, Winchester, Remington, Buffalo Bore and others. Basically, the 125 gr full house travels faster at 1500 fps to 1600 fps than a 158 gr round in a 4 inch barrel revolver. Back in the days when LE used a revolver this round was developed as a improvement to the 158 gr round. Experts in the industry claims this 125 gr round is the best manstropper still availiable. This includes all the rounds availiable for semi-autos. Whether this is true are not anymore is unclear. From my experience in my 357s the round has a lot of blast and recoil. I only have full size revolvers. Plus this round is much harder on a revolver than the heavier grain rounds. Any revolver that has seen a lot of this round can if not taken care of and cleaned on a regular basis wear the forcing cone out sooner. In your 686 you will be ok but there have been reports on various forums of this round causing problems even with heavier frame revolvers.
Bottom line: The round is a great SD round. But in my opinion I would not shoot the round all the time. Too me the heavier grain rounds are better for the range and most shooting situations.
Here is part of a post that explains the affects of the lighter grain full house 357s.
The lighter bullet causes a few things to happen.
1: It accelerates faster in the cylinder, striking the forcing cone MUCH harder than the 158gr bullet.
2: The shorter 125gr bullet leaves the case before a 158gr bullet, causing more unburned powder to fly forward and combust in the throat and barrel. This causes flamecutting on the topstrap and peening of the forcing cone. Again, very bad.
3: The recoil impulse of the 125gr loads are much sharper and severe than a comparable 158gr load, so it batters the gun HARD.
4: To be honest, the 125gr load is the most common out there, but it is not the best load. It is light and fast and while it expands violently, it tends to underpenetrate. The 158gr expands and has enough momentum to smash through and hit vitals.
There is more infomation on the net but the above paragraph I feel explains the round well.
Good luck and show us some pictures of your 686.
Howard