AKA, “Yet another reason to have a small lathe.”
Obligatory disclaimer: We don’t guarantee that these will be safe in your rifle or with your components. Do this at your own risk.
The cartridge cases converted using this method are NOT safe to use with full loads. Read that again. They are only safe to use with low pressure reduced loads. Use at your own risk.
My friend N. has recently delved into shooting his M-1891/59 Mosin-Nagant and No.4 Mk.I Lee-Enfield with "mousefart" loads. These are even less powerful than Ed Harris’s “The Load” of 13 grains of Red Dot under a 150 - 180 grain bullet. N. is loading a 115 or 125 grain cast bullet on top of 5.0 grains of Bullseye. (His first try in 7.62R used 6 grains of Bullseye but accuracy was horrible. Backing off to 5 grains shrank the groups.)
He has a good stock of No.209 shotshell primers but not a lot of large rifle primers. He also has a lathe in his basement workshop, as you can see in the picture. It’s an older Jet unit with a 7” swing (not sure how long it is). He took some Berdan primed .303 cases that he’d stashed away and drilled out the primer pockets so they will accept No.209 primers.
Per N., he used three drill bits to modify each case:
"Center drill to keep main drill from following the firing pin dent, letter C for the main body, 21/64 to countersink the flange on the battery cup. Be careful! My Fiocchi primers are .002" larger than my Cheddite, and I understand the Cheddites are larger than most Yankee brands."
So measure the No.209 primers you have and pick drill sizes to match.
We haven't tried this with Boxer-primed brass but I don't see why it wouldn't work. This project was primarily to make something useful from what otherwise would be trash.
Something else to consider is how your rifle will handle escaping gas if something goes wrong. I have been present when someone experienced complete case head separations in a No.4 Lee-Enfield and another time in a Mosin-Nagant. The Lee-Enfield shooter didn't notice anything awry until he opened the bolt. The Mosin shooter got gas back in his face but was uninjured because he wore eye protection.
Also note that a lot of milsurp .303 and all 7.62x54R were corrosively primed. Make sure to clean your rifle accordingly when shooting that ammo. You also need to rinse out the brass with water if you plan to reuse it. (The No.209 primers are not corrosive. I'm only referring to the initial firing with the original primers.)