I am not a fan of the belted magnum cartridges. One of the first problems you run into is the issue that base to shoulder dimensions are not standardized. Only base to belt is standardized and that is the head space. Base to shoulder can be all over the place and thus you need gauges to control this dimension. I purchased the Wilson Adjustable Case Gage
http://www.lewilson.com/adjustablecasegage.html for my 375 H&H. Though I could have used a Sinclair Comparator on setting up the dies for my 300 H&H. This page shows how to use the Wilson adjustable case gage.:
Extending Cartridge Case Life
http://www.realguns.com/Commentary/comar46.htm
If you size your cases without measuring just how much you are pushing the shoulder back, your case life will be short. Maybe three reloads. Because I did not want my $2.00 a pop 300 H&H cases to stretch excessively on the first firing, I lubricated the cases before firing. I either rubbed on paste wax, which melts and acts as a lubricant, or rubbed case lube on the loaded cartridges. This prevented the front of the case sticking to the chamber, the case slide to the bolt face without sidewall stretch, and the shoulders folded out to the chamber walls. After first firing, I think I used a Sinclair Comparator to set up the die. I did not want to push the shoulder back more than 0.003", and I did not want a crush fit case either. I only have about four reloads on my 300 H&H cases but there is absolutely no evidence of case head separation. I checked with a bent paper clip, as the article shows how to do.
As for a collet die, are you thinking of this collet die?
Reloading Belted Magnum Calibers
http://www.larrywillis.com/windex.html
The merchant has a lot of comments on reloading belted magnums:
Reloading Belted Magnums
http://www.larrywillis.com/answers.html
I suspect if there is a dimensional issue between the chamber and sizing die, this collet die may be needed. I have not encountered this (yet) but Larry Willis must have, and he has come up with a solution. I would get something like after I had chambering problems.
Another issue I ran into, which reinforces my dislike of belted magnums, was rim lock. Firing 235 grain bullets in my 375 H&H, the cases slide around in the magazine. The top round, the edge of the belt caught on the rim of the next round down. This prevented me from closing the bolt as I had "rim lock". I had to pull the bolt back, push on the stack with my thumb, to clear the rim lock, before I could get the cartridge to feed. I think for a cartridge whose primary purpose is hunting big game, the type that could crush you into a mass of hamburger meat, or shred you with their teeth, this is not a good case design. I would say, anyone hunting large game with one of these rifles ought to have a big stiff spear to grab incase the bolt locks up. Going retro with a Paleolithic spear could save your life.! I have not had this problem with my 300 H&H and I don't know why. Maybe the recoil is less, maybe the cases have less space to slide around. My 300 H&H is an incredibly smooth feeding rifle as the cartridge is very tapered.
I don't fire my 375 H&H all that much, maybe have five reloads on one box. There are elephants around, but they all hide out in Bryant–Denny Stadium on the weekends.
https://en.wikipedia.org/wiki/Bryant–Denny_Stadium