If you haven't shot a handgun in 30 years you should definately start out with the 38 spl. Typically the 38 spl. will produce in the range of 200 to 210 ft.lbs. of muzzle energy. Most commercial loads for 357 Magnum range ammo run about 600 ft.lbs. of muzzle energy. This means the recoil with the Magnum will be about TRIPLE that of the 38 spl. So, it's an excellent idea to spend a bit of time getting acclimated with the milder caliber.
When you think your ready to try the Magnums, start out with just one round loaded in the cylinder. That way you'll get a good idea of how much recoil you have to contend with and how much more firmly you need to hold the gun when shooting this round. Once you know what your dealing with, you can then load up and have some fun.
BTW, I'm a wimp when it comes to the Magnums, my model 620 now has the grip for the 500 Magnum mounted on it. When I first got the 620 I tried it with the tiny rubber grips it came with and it only took 3 loads of the cylinder to convince me I needed a larger grip with a bit more padding. Now, it's just a big grin to run some Magnums downrange with no real pain involved.
I can also tell you that the 357 Magnum is not an easy caliber to master, even with well fitting grips it still hits hard enough to let you know you're dealing with a real powerhouse. It also hits hard enough to promote a flinch if you overdo it, if you find your accuracy tanking, it's time to call it a day. Finally, it's LOUD, I suggest that you double up on the hearing protection and use both ear plugs AND over the ear muffs when shooting the Magnums.
You will also want to familiarize yourself with the difference between the Round butt and Square butt grip frame. Your 6 inch 686 could have either style of grip frame depending on when it was made. You'll need to know this if you decide you want to try a different grip. Note, the 500 Magnum grip I mentioned will ONLY fit the Round butt grip frame, which is typical to the newer 686's. Take a look at the following link, it provides pictures of the two different style grip frames.
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