I know this die can be custom ordered from Lee, but wondered if you could use the .416 Remington version by not seating it into the press as deep. I'm doubting it, but thought I'd ask.

I asked Lee that question and they told me no. Of course, they are trying to sell a custom Rigby crimp die at a high price. I just use the crimp feature that is included in my Lyman seating die. Since I don't load a whole lot of 416 Rigby's, I do it as a two step process. First seat to length without crimping then back off your seating stem and adjust the die to crimp. Works fine for me. If you don't want to do the two step process, buy just a seating die body from Lyman and use that as a crimping die. Alot cheaper than Lee's custom crimp die.

The 2 cartridge lengths must be with in .050" of each other to work. If i remember correctly?? Check Lee's FAQ website, its on there.

You can modify a Lee FCD to work with a shorter round by removing material from the bottom of the collet, or adding a spacer that fits over the case on top of the shell holder to work with a longer round, I use a spacer on a 45-70 FCD to work on 45-120.

I looked at this when I had an 8x60S rifle, if I could sue an 8x57 Lee Factory Crimp.

As far as I saw the crimp is operated by the top of the shellholder contacting it.

Therefore, as said, you could maybe braze a "ring of steel" to the base the exact extra length of a longer cartridge.

But if wanting to alter it for a shorter cartridge I would have to look at one again.

Have,nt personaly tested the theory, but since contact with the shell holder controls amount of crimp applied, you could do it in a backwards fashion..
Set your crimp, use a spacer, say a 45 cal case,dill out the bottom of the case, which will sit on top of the collet, keeping things relatively sqared.. Cut the case a little proud of the correct depth, file down to fine tune your seating depth to adjust..

In theory, should oughta work.
In practice, I don,t have enough patience to do that myself. Sent Lee a money order and a pilot cartridge.
Course, if you happen to have that stuff hangin around already, you could test the theory, see if it works to your satisfaction.
Just a thought...