I've been loading the Berry's 124gr FP and like them a lot. I pretty much always load Berrys but have had to scrounge my starting data by looking at similar bullets and using online recipes from TFL and others. It's hard to know what on line data you can trust but if you hit enough sources, you start to see a pattern of what's low, high and the range in the middle.
For the 124gr FP, (I don't use the hollow back but the front profile is I think the same), you have to seat them much deeper for them to seat correctly in the barrel. The FP shortens the bullet and therefore the OAL as well and the FP also have a sharper shoulder to clear. If I want them to work in every gun, I have to go to less than 1.100 OAL (1.106 - 1.108). That's pretty short. If I for go the 5906, then I can get by with 1.120 for an OAL which isn't bad, it's only a little less internal volume than I have when I use the 115gr RN from Berrys (seated to about 1.152).
I target 1.116 OAL (+/- .002). I started with Unique around 4.8gr but wasn't 100% happy (although I love it for the 115gr). So I switched to Power Pistol and really like the results. I've been using 5.7gr of Power Pistol which is a bit on the heavy side but I am looking for punch when I'm shooting the 124gr FP (otherwise I'd just shoot the 115's). I'd back off quite a bit when starting. Of course Titegroup will be different yet (not sure how it relates to Power Pistol or Unique).
597 VTR, because there's so many cans and so little time!