IMO, I believe our OP needs a taper crimp die (or similar product).
I disagree. I don't think the OP
needs anything more than the standard die set and knowing how to use it.
Not saying there isn't a use for a taper crimp die or something similar, only that there isn't a
need to go buy another die.
There are a lot of possible variations to loading techniques that will handle almost every combination of bullet and desired crimp. Most can be done with the standard dies and their usual roll crimp.
What the OP really needs to do, for specific help and suggestions is to tell us what gun(s) he's using, and what bullet type he actually has. "plated 125gr" can be a couple of different things. RN?, SWC? Conical?? is there a crimp groove? Or a shoulder that could be crimped over?
I've got some plated 125s that are SWC types clearly intended for revolver use. I've had other plated 125s that were RN types meant for semi autos (9mm).
Both types can be successfully used in revolvers, up to their designed speed limits, BUT the loading & crimping process is slightly different.
So, don't go out and buy this or that ..right now, tell us what you've got, and what you want to do with it. Odds are real good someone here has experience with something similar.
I've got .357 Mag DA and SA revolvers, a couple of semi autos, a single shot break action (Contender) and a Marlin carbine. Been loading .357 since the early 1970s, and I've done (or heard of) a number of oddball things. I don't doubt there other folks here with even larger experience bases. TO give you good specific answers, we need good specific information. Give us that, odds are good we'll have something useful for you.