HOMEIn the dimension along the sides of the anvil, the primers all measured 0.3150" or 0.3155".
In the dimension across the ends of the anvil (without touching any of the anvil) the primers all measured 0.3175" or 0.3180".
Here's a spot for everything about .50BMG or "Extra-Large Rifle" primers.
Some loading manuals will tell you that there are four types of primers (ignoring shotguns): Large Rifle, Small Rifle, Large Pistol, and Small Pistol. For a lot of reloaders that's effectively true, but the .50 takes its' own primer, referred to as '50BMG' or "Extra-Large Rifle".
Commerically available 50BMG primers are made by IMI, RWS, and CCI. Here in the U.S. the CCI is the most available by a wide margin, though competitive shooters seem to prefer the RWS primers when they can be obtained.
The picture at the top of the page shows CCI primers. A "Large Rifle" primer is shown next to the 50BMG primers for comparison. The trays are just like their smaller counterparts, only bigger and still contain 100 primers. The cartons (for CCI, at least) contain only 5 trays (or 500 primers) unlike their smaller cousins which come in cartons of 10 trays (1000 primers).
CCI primers are oval!
Not only are they oval, they are very consistently so. I was trying to measure a set of them, to generate some 'typical' numbers for this page (see below) and discovered that all my primers were oval, and measured within 0.0005" of each other. (That's one-half of one-thousandth of an inch - pretty darn close.)
Notice the orientation of the primer anvils in the picture above - these measurements were very consistent primer-to-primer.
I don't have any other brands to examine, but would be interested in data from RWS and IMI.
'Typical' primer pocket dimensions
Note that primer pocket dimensions refer to the cartridge case, not the primer.
These aren't any kind of official spec, but some typical pocket dimensions are:
If I can find a primer spec or anything, I'll add it...