I had a lot of problems trying to find shafts of the right diameter to fit various (berdan) cases that I wanted to experiment with. (READ: total frustration!)
I came up with this solution which a local machine shop made for me ($60 cash for both cylinder & piston)
I now use a brass cylinder that any case of 0.500" diameter or less fits completely inside like a "cup". (the brass rod was salvaged from a "damaged propeller shaft")
There is a centered hole in the bottom of the "cup" about 0.250" diameter to eject the primer.
There is a relief channel at the very bottom edge of the cup from the 1/4" ejector hole to the outside wall of the "cup". this allows water and primer to gush out.
I have a stainless steel rod ("piston") (0.499" dia) with a lip on one end to limit its travel into the cylinder.
The length of the cavity inside of the brass cylinder with piston fully inserted is longer that the longest case that I might want to de-prime.
I stand the cylinder upright (primer hole down), drop in a case (usually Swiss 7.5x55 GP11) pour in water to the brim and insert the S.S. piston.
One good SMACK with the rubber mallet and the primer is GONZO! Any and all primers - live ones, dead ones, crimped ones, Berdan & Boxer. Never had a failure. Ever.
The nicest thing about this method is that there is never any damage to the case because nothing touches it but water.
My wife makes me do this in the garage on the concrete floor by the floor drain (OR ELSE!)
I have a pdf drawing of this device as submitted to my local machine shop. If anybody wants a free copy, PM me with your email address and I'll send it to you...
And if you need either type of Berdan Primers, talk to Henry at Budget Shooter Supply - he is THE MAN!