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A 4.2 inch barrel is pretty ideal in in squeezing out about 90% of the ballistic performance of the .357 Mag while allowing for a not unreasonable carry piece.
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The .357Mag is one handgun caliber that really does benefit from a couple more inches over 4".
Looking at the BBTI data, going from 4" to 6" nets you, on average, over 200fps.
You don't always get a lot of bang for the buck going from 4" to 6" in a pistol caliber, but the .357Mag makes it pay.
For comparison, based on the BBTI data, going from 4" to 6" in a 9mm, buys you, on average, a little less than 130fps.
I don't know how to calculate what percentage that is (e.g. 90%) because you'd have to pick a length that offers 100% performance for reference. If we do that--and assume that an 8" barrel is 100% performance in .357Mag, then 4" gives you 82% performance and 6% gives you 93% based on the BBTI data.