got one case measured this morning. It is .215"
I'm thinking that 5.5mm is correct. That would be .002" interference fit.
One thing you may want to take a serious look at, before you order a bunch of Berdan primers is COST.
I have 3 different sizes of Berdan primers on hand (no, they aren't for sale). I purchased one sleeve from my local gun shop that had them on the shelf for a few years, another sleeve from Canada Ammo and the third from North Sylva, who seem to have different sizes in stock, or at least they did at one time.
I keep them on hand specifically for cases I can't find Boxer cases for, such as Kropatschek 8x60R and other obsolete/obscure cartridges.
The GP11 cases are lovely but IMHO not worth the effort or expense of reloading.
When I purchased the primers, close to ten years ago for GP11, they were around $25 per 100 plus taxes and shipping. 7.5x55 Boxer brass was almost unobtainium unless you could find some from Norma.
Now, there are lots of shops that carry both the ammo and brass at reasonable cost.
The cheapest Berdan primers came out of Canada Ammo as part of their Dominion line, $35/1000. They didn't fit the GP11 primer pockets.