Hi James13, and welcome to TFL. We're here to help where we can.
Lots of good replies so far, but I think some of the explanations may assume you know more than a new loader does. So I'm going to remove all doubt an get into some fundamentals.
First, there are dies that roll crimp; and dies that taper crimp. If you walk into a store that sells reloading gear, and you ask for a 38/357 die set, they are going to sell you a set with a roll crimp die.
Historically, revolver cartridges used bullets what were roll crimped to secure them. Subsequently, revolver bullets had either a crimp groove (usually lead slugs) or a cannelure (usually jacketed bullets). You'd seat the bullet to where the case mouth was aligned with the groove/cannelure, and then you'd crimp them.
Times have changed. And now we have plated bullets. And most plated bullets have no crimp groove or cannelure (some do). Ideally, these bullets should be taper crimped - like semi-auto cartridges. (Plated bullets - whether they have a groove/cannelure or not - should be taper crimped; but that's outside the scope of your problem.)
Taper crimping is a far less harsh treatment. And it's more forgiving with variations in brass length (which you suspect you might have - a good time to measure them, btw).
I have used a lot of Berry's (and X-treme) 125 FP's. So I am quite familiar with them. However, I only use them for 38 Special. I have never attempted to roll crimp a plated 125 in 357 Mag (or 38), but I suspect I would not crush/buckle the case in the process. That said, my gut is telling me that there's more going on here than simply using a roll crimp die, as opposed to a taper. But I don't know what - it's just my gut feeling, backed by 33 years of loading experience.
Either way, if you plan on using a lot of plated bullets, I strongly recommend investing in a taper crimp die for your 38/357. If you do that, your dilemma may evaporate (albeit, not necessarily "solved").
Which puts us at the next level (and a little off-subject, if I may) . . .
Can you tell us a little bit about your load purpose for these 357/125's? Plated bullets are generally for making basic "range shooter" ammo. Can you tell us your charge weight and propellant used? While you're at it, what gun will be shooting these? Most importantly, the barrel length?
Plated bullets are excellent - I load/shoot a lot of them and find them to be a clean alternative to lead bullets, without the expense of jacketed. Notice, I did not say I used them as a low cost substitute for jacketed - they're not. And trying to use them as such is where problems arise. That is not their purpose - as much as so many loaders try.
I load my 125 plated FP's as 38 Specials (one of my very favorite range rounds, btw). The reason I do this is because loading them as 357 Magnum - even fairly light - is pushing the envelope of a plated bullets' weak construction.
My lowest power 357 Magnum loading does use a plated bullet (X-treme 158's); but I'm not driving them hard at all - probably just a little above 38 Special +P pressures. Every 357 Mag loading above that gets a jacketed bullet. I mention this because it is my opinion that plated bullets are very well suited for 38 Special applications; but can't really hold up to the type of pressures normally associated with 357 Magnum. I'm curious where you are with all this.