I don't usually need one and I load a lot of belted magnums.
But I made my own, if can run a lathe or know anyone who can...this is fast and simple.
I use a lot of range brass fired in every type of rifle with different brands .
This is a free to me homemade die, size .5095" with spring back about. 5105"
My full length sizing die does a good job, but a few of these are like .513" so run em through the die and get, .5105". Careful set up so ya don't hit the belt can be felt in the press...so easy after FL sizing...just run through no lube required.
Mine is precisely made and polished to .5095" with a small .005" radius.
But lee makes cheap bullet sizing dies, probably, .510" cut the champher off and polish a small radius, may help ya out ...50BMG has expensive C& H .5095" bullet sizing dies but they need the champher removed, cut with carbide tools, as its hardened steel...and they are almost never in stock.