Cartridge is from Ammo Manufacture:  Winchester .357 Mag

Muzzle Velocity; Advertised:   1295 fps  (No barrel length mentioned)     
Muzzle Energy; Advertised:    410 ft/lbs


                                               
Actual Instrumental Velocity:   
2½" barrel 66:     1185 fps  42°F
4" barrel 686:        1260 fps  42°F
5" barrel 686 plus PC:     1322 fps  43°F




Brass Make/Headstamp:  Winchester 357 MAG
Bullet Make/Weight/Construction/Info:  Winchester 110grs JHP







.3566" dia. 
.529" long.
Actual Weight:     109.8grs.

C.O.A.L.:    1.563"
Primer:  Winchester WSP
Case: Brass
1.282" long
79.8grs w/o primer



Powder Description/Positive ID/Type/Charge Weight:  Flattened Ball;  8.8grs.








When shooting the 5" barrel group, there was one unusually slow round. Had that not occurred the average would have been closer to 1350 fps.

Shot another group with the 5" barrel today, 2-6.  It was colder today and the velocity reflects it.  One flyer, but tighter Sd though.

RDub I would suggest HS6 as the powder...

I don't have any HS-6 currently, but I do have some 540, which is supposed to be the same thing.
Here is the factory powder;



Here is 540;



540 looks more uniform and more flattened than the factory powder..

When I study the Hodgdon data for the 38 spl and +P using HS6 it seems to fit even though these are loaded in 357 Mag
Hodgdon shows the 38spl +P with 110 XTP using HS6 8.4 grains making 1268 fps tested in a 7.7" barrel. 
This could be very close what you might get with the factory loading using 8.8 grains of HS6.
Thanks  again for sharing!

Data for 110 grain JHP using Winchester 540 powder with 38 spl +P shown is 8.7 grains @ 1070 fps  (Winchester 2014 data)

Shot another group with the 5" gun today.  See the original post.. One flyer but the Sd is tighter.