IF you were to build a bench rest class rifle in .357 Magnum, you might, MIGHT, be able to find some measurable difference in accuracy between .357 Magnum and .38 Special, but I doubt it. AND, even if you did, it would ONLY apply that that one specific rifle you used.
The stories have been around probably as long as the magnum, that the "long jump" due to the longer chamber means the shorter .38 Spl must be less accurate. Nobody has yet provided any proof that is the case, beyond the possible idiosyncrasies of a specific gun.
That "long jump" is based on the difference in case length, which is 0.135".
This completely ignores the fact that while the .357 case is .135" longer than the .38 case, the difference in the (max) length of the loaded rounds is 0.040"
AND a difference of 0.040" is only a fraction of the distance the bullet has to go to reach the rifling in every revolver, and I would say it is also an insignificant difference in a rifle as well.
In a revolver or any regular lever gun chambered in .357 Magnum, shooting .38 specials has never resulted in any SIGNIFICANT difference in accuracy that I've ever experienced or heard of.
IF someone claims the .38 special, fired in a .357 Magnum chamber is less accurate because the .38 Special is shorter, I'd ask them to prove it.
I'd be willing to make a wager they cannot.