The short version, the l & k frame 357mag cylinders are longer then the 27/627/28 cylinders.
The somewhat longer version:
S&W made their 27 with pinned bbl's and recessed cylinder until 1981. Then they switched to a "crush" fit bbl and standard cylinders.
Recessed ='s 1.62"
non-recessed ='s 1.57"
What the big deal in the cylinder change? The recessed cylinders supported the cases better for full house loads.
The k/l frame revolvers use a 1.62" cylinder that is not recessed.
1.620" - 1.570" ='s 50/1000th's
Then add to the mix that s&w got the bright idea "Why not use the extra cylinder diameter on the n-frame???"
j-frame ='s 1.446/five shot max
l-frame ='s 1.559"/seven shot max
n-frame ='s 1.710"/eight shot max
Then just for the heck of it the got back to the recessed/non-recessed cylinders, moon clip time. The easiest way to tell if a 627 cylinder is recessed is to look at the amount of bbl sticking out the frame. A link to 2 different n-frame cylinder's fluted vs non-flutted, note the bbl lengths inside the frame in front of the cylinder.
http://www.personaldefenseworld.com/2015/09/enhanced-snubbies-smith-wesson-model-627-327-revolvers/
At the end of the day all n-frame cylinders are not created equal. A 44mag clyinder is a lot longer then it's 357 counterpart. Same frame, same cylinder diameter, different lengths due to performance & accuracy from the difference in max saami oal's/bullet jump. 357 1.590" vs 44mag 1.610".
On a side note:
This is why the model and dash #'s are important when looking for replacement parts. Different cylinder lengths ='s different bbl shoulder lengths. Add to the the +/- .003" specs for the cylinders gap. The recommended "target #) bbl gap is 3/1000th's, for every 1/1000th's over that you loose 10fps for every 1,1000th's. Used to love the older 27's with the fully supported cylinders (1.620" recessed) and the pinned bbl's. Would set them up with 2/1000th's cylinder gap, couple that with the shorter freebore of the cylinders and the end result is one heck of a shooter.
Something else to think about, the dan wesson 14,15,715's have
recessed cylinders
bbl gaps that can be set from 1/1000th to 7/1000th
Tension free bbl's.