I have 686-8 6" which has a stainless barrel I guess (also the inside of the barrel?). Is such a barrel as strong as a barrel in carbon steel?

The trigger parts are made of carbon steel because a stronger material is required than is possible in stainless? .....or?

How many .357 FMJ factory loads can a barrel take before it is not as accurate as when is left the factory? 50.000?

Typical stainless steel used to make firearm barrels is type 416 stainless. Less common but also used is type 436 stainless. Both of these steels have mechanical properties very similar of 4140 drawn steel (chrome moly) and can be heat treated using nearly identical methods. So, yeah the stainless steels used for the barrel in your 686 is quite strong and likely just as strong as a carbon steel barrel.

I'll also note that what you give up to gain that strenghth is that the 400 series stainless steels are not nearly as corrosion resistant as the type 300 stainless steels. One thing that S&W does to try and enhance the corrosion resistance of their stainless revolvers is "passivate" the surface by dipping it in an acid that basically disolves the raw iron out of the surface. This results in a thin "skin" that has a higher chromium content similar to a type 300 stainless with the underlying material retaining it's full alloy and strength.

According to Jim Carmichael in the book 'The Modern Rifle' (great book from the 70's BTW), regarding stainless vs carbon barrel wear, stainless has an advantage in that the lower carbon content in the stainless alloy results in less surface 'carbon burn-out' (i.e. erosion) in the throat and thus longer barrel life. This is of course in regard to rifle barrels, but I'd think it also applies to handgun barrels as well.

It sure is more resilient to ham-fisted handling and cleaning methods too

Dave

OK!
Thank you for all this information. I will continue using .357 FMJ for target shooting and hope it can take many x1000 shots with good accuracy! ......I have to get used to the kick.......