I was going to use 5 grains of Titegroup, CCI small pistol primers pushing a 125 gr. Winchester JHP. It's listed as a 38 special +P load on the Hodgdon Load Data website. Think it will be OK in the 357 case?
Could the small bit of extra air space in a 357 case make it dangerous? If so, 7.5 grains of Titegroup is listed as a max 357 load. I have used 7.4 grains with regular small pistol primers without issue in the 357 case. Would it be better to load 6 grains? I'm just going to use the loads for plinking/target practice.
Titegroup is a modern powder that was designed to not be "position sensitive" and have problems with light loadings in large cases. Specifically, Hodgdon says that Titegroup was "designed for accuracy" and that "unlike pistol powders of the past, powder position in large cases (45 Colt, 357 Magnum and others) has virtually no effect on velocity and performance." Quoted from here.
According to Hogdgon's load data, 4.3 is the starting Titegroup load in a .38 case using a 125 Hornady XTP JHP. They list 6.8 as the starting load in a .357 case with the same bullet, but that is already giving over 1400 fps.
Notably, the starting load in a .357 case with Titegroup is only 4.0 grains with a 125 grain LRN and only 3.0 grains with a 90 grain LRN. This would seem to indicate that small charges of Titegroup are fine in the .357 case.
It is disappointing that they don't list a lower starting number with the 125 JHP, as you can't be the first person that wanted to just plink with them, and I find it hard to believe Titegroup can do all these different things but can't go less than 1400fps with a 125 JHP. Being that they don't list it though, it is difficult to recommend trying it. However, looking at all of the other factors.... Well, it's up to you, but it is far from the riskiest reloading practice I have ever heard of, for what that's worth. Said another way, I'd be willing to bet it would be fine starting with somewhere around 5 or so grains. A rough rule of thumb is that jacketed bullets need .5 - 1.0 grain more fast burning pistol powder than similar weight lead for roughly similar velocities and pressures. Again, a rough rule of thumb. (Be careful with these, or you could lose a real thumb or worse!). Anyway, based on that, I would hazard a guess that something in the vicinity of 5.0 - 5.5 would be ok. No guarantees or warranties express or implied. Reload at your own risk, and please be careful!
I've got plenty of 38 cases already loaded up and ready to go. I just have an extra 70-80 357 cases that I don't want to load up to full power since I've already got 550 magnums loaded. I use the 38's to plink around/practice with. The milder 38's are a little easier on the model 66 S&W.