Firstly a disclaimer: this is how I have converted Berdan primed brass to accept Boxer type primers. It’s probably not be the best way, I’m sure its not the only, but it is the way I have done it.
I only really ever did this conversion to see if I could, I am a hopeless tinkerer at heart.
Boxer primers only come in a few sizes, but Berdan primers are from the good old days when anyone could and did make what ever they wanted.
Old military Berdan .303 brass is usually a bigger primer than large rifle primers.
A converted Berdan case on the left, modern Boxer case on the right, showing the size difference in the two primer pockets. This was the point I found out about the different sized primer pockets and realised that converting that brass was futile beyond the practice.
I have read about people tapping the primer pocket and screwing in a sleeve with the correct sized pocket, but I didn’t see it being worth that much time or effort to make some .303’s.
Things you’ll need:
Some Berdan primed brass. (preferably with a primer pocket that will take common Boxer primer once you’re done)
A 4mm & 2mm drill bit.
I use a cheap ebay ottoscope to look inside the cases to see if they’re Berdan primed, but you don’t really need it.
I used a mini lathe (much, much easier), you can do it with a drill press but its a lot more fiddly getting everything straight and even.
A nail or screwdriver or something to pick the Berdan primer out with.
The Ottoscope.
My little lathe with a cuppa for size comparison.
First thing to do after establishing that your brass needs converting is the place the shell into the chuck of the lathe (or set it up in a vice on the drill press) and drill into it primer with the 4mm drill bit, you only need to drill in a couple of mm.
Then using a nail or a small screwdriver to pick out whats left of the primer. I couldn’t get a pic of me doing it but there’s not really much to it.
Next chuck the brass back into the chuck and use the 4mm drill bit to drill away the anvil (small post in the centre of the primer pocket).
Then swap out the drill bit for the 2mm bit and drill right through the centre of the primer pocket to create the new flash hole!
Using the 4mm drill bit swage the flash hole by hand to clean away any burs and you now have brass ready to be boxer primed and reloaded.
Your crazy 
