right out of the 2008 free fer nuthin' Hodgon Data Manual:

38 Special - 158 gr lead SWC - 4.2 gr - 804 ft/sec - 13,700 CUP
357 Mag - 158 gr lead SWC - 4.2 gr - 865 ft/sec - 20,400 CUP

I don't usually expect to see the same bullet & powder charge in a larger case give faster results.
..

There's just not enough information there to figure out why, but I would guess it is the primer used and/or the barrel length used to test each load. I've seen stranger things in reloading books, that's for sure.

Hello Two Dot.
That data may just be right on. Some variables are: They fired the 38s in a 357 Chamber. Different brands of cases used. Different cyl. gaps, if any, in their pressure gun.
* Some magnum cases have the same volume as the std. 38s do. They are made with thicker brass to hold the higher pressures. And I'm thinking this Co. wouldn't scrimp with buying cheap handgun brass for their testing. As the previous reply noted, they well may prime hundreds of magnum cases at a time with the hotter primers. It's pretty much of a given, that a ballistic lab, will be using streamlined procedures, as they are all working on the clock. Carpooler

50% more pressure with a larger? case capacity?? I guess so.

Call them and ask them

The info I've seen for using trailboss in the 357 mag is they recommend using magnum primers. Thats the only caliber with that recommendation. Still seems like a big pressure jump. But maybe not.

Adding fillers change pressures, why no larger volume cases? Look at 45-70 Trailboss reloads. Same load for 4 different bullet weights for 300 grain to 385 grain! Trapdoor load. 325 grain Levereveloution in the Contender hurt!!!!

So long as the load is NOT compressed there is a quite wide
'range' of loads w/ Trail Boss.

Fill to base of seated bullet for max(UNcompressed powder)-2/3 of that weight for minimum..

Going by their online information the 357 load uses Winchester small pistol magnum primers which tend to be hotter than even other pistol magnum primers in my experience.
My guess it that is where the extra velocity comes from.

Don't believe everything you read in reloading books. One of the better reloading books IMO is the 7th Edition of the Hornady Manual. However, if you look at the loading for the 35 Rem and the 358 Win I am sure that you'll realize they got a couple of pages mixed up. The 35 Rem with the Marlin 336 and 20 " barrel and 200 grain bullets is shooting 2900FPS.
The next page look at what they have for the 358 Winchester in a T/C Encore and 24" barrel and 200 grain bullets 2600 fps.

If you believe that I got a deal for you.