The "correct" replacement primer for these cases, and anything else with a .250" Berdan primer is the RWS 6000. These have been akin to hens teeth for a couple of decades.
The next problem is the brass itself. The Cordite sticks were not poked in there individually by small children, but inserted BEFORE the case was finally formed, i.e., while it was still a cylinder with a rim. This, in turn, is a hangover from the very first .303 production which used a SOLID PELLET of compressed Black Powder. See also the previous service cartridge, the .577 / .450 for the Martini Henry. Same basic production method. Delete BP "pellet" and insert, literally, a "bundle" of cordite sticks, AND the "glazeboard" wad.
The wad wasn't just there to stop the Cordite rattling around. It provided a "platform" for the bullet to sit on for the next few operations in completing the round. The final body taper, neck and shoulder were then formed in a succession of dies.
This, of course meant that the brass was NOT annealed after final forming, for fairly obvious reasons.. Thus, all of the stresses generated by these final stages remained in the thinnest regions of the brass. Hence neck and shoulder cracks that appear in "New Old Stock" .303 ammo made this way. Furthermore, there was no "neck crimp" as we know it today, but a few "stab-crimps" about halfway down the neck. These are supposed to line up with the cannelure on the (Mk-7) bullet.
There was also supposed to be a small amount of a bituminous sealer on the bullet before the neck was formed onto it. The use of a Mercury-based primer was the final nail in the coffin for the cases. Whilst the primer composition was sealed in its little copper cup, it posed no problems. Upon firing (
POF
optional), the mercuric Fulminate generated a serious flash which touched off the Cordite. Thus:
The metallic Mercury reloaded in the process was blasted into the walls of the brass case. This seals the ultimate doom of the case. Mercury disrupts the Copper / Zinc bond that makes "brass" such a versatile material. The sting in the tail with these primers is the large amount, relatively speaking), of CORROSIVE Chlorate / Chloride salts released. These have little or no effect on the brass case, but are murder on other things, like, say, barrels.
VERY few .303 barrels get to be "SHOT out", i.e., throats, lands and muzzle "washed out" by high-temperature propellant gases. Most die a lingering death from corrosion, primarily caused by lack of correct and thorough cleaning.
IF you can find a stash of those RWS 6000 primers, or an equivalent, reloading is perfectly doable, once you have successfully removed the old primer without mangling the anvil in the primer pocket, and having removed the inevitable remains of the primer crimp seal.. I'm down to my last few hundred of them and they get used in once-fired cases that will be reloaded for "bush" work and NOT recovered.
Back in the "good-old-days" here in Oz, it was a relatively common practice for "enthusiasts" to strip the rather plentiful "surplus" Mk-7 ball down and remove the Mercuric primer with a Hydraulic gizmo. The "clean" cases were THEN annealed, at least by the truly dedicated, (fifty years too late). A lot of the time the cases were reformed into sundry "wildcat" configurations; .22, .25, being the most popular. The .270-.303 was not a roaring success, probably because almost all of the bullets available in those time were of US manufacture and designed to "work" at .270Win velocities.
In short: Remove the bullets and load them into new, Boxer-primed cases using nice new propellants and use as range ammo. Be aware that some lots of POF have "MUCH lighter-than-normal" bullets and these are of VERY dubious accuracy.
PM
Originally Posted by speckles
MK7 loading have been the most accurate ammo I've been able to obtain, although some 
