I assume they are. For dies, you'd need a few fired cases. I'd have Hornady or Lee make the dies for me. The reamer should be avialable from Pacific.
: One need only ask for a reamer for the Improved .303 necked up to .375. If there are other than standard dimensions desired, that would also be passed on, such as .001" over head size for new unfired .303 brass. If other than standard neck length or throat was desired, etc.
; It would be best to pick a manufacture of brass, buy 300 of them (more or less), such as Federal, WW or RP. I'd personally go for Federal brass. Measure the head size, add .001" and order the reamer to that size. I'd pick .440" as the shoulder diameter, with a standard neck length and 40 degree shoulder, leaving the shoulder placement to Pacific as they'll get it right. The shoulder diameter I listed will give .008" taper per side, which will work perfectly. The head diameter would be .458".
: BTW - I have made bullets for my .458 2" from shortened .303 brass, filled with a lead core and formed over into a round nose - excellent penetration or expansion, depending on the alloy of the core - good accuracy too, same as store bought bullets. I can see why John Buhmiller and Harold Johnson used them.
: The reulting .375/.303 case should give around 2,300fps to 2,400fps with 270gr. RN's or spitzers and would make a very nice big game rifle for B.C. or Alaska. At around 2,500fps or a bit more, with a 235gr. Barnes XTS, one could make do with one bullet for all game.
; To form brass, one merely necks up .303 brass using a tapered .44 mag expander, then size the neck down using other dies in the junk drawer, to get the shoulder in the right place. One needs to fire 3 of these so they fit the new chamber then send the cases off so dies can be made. Another method is to load up about 10gr. of bullseye, fill the .303 case with cream of wheat, and fire it into a trash can(steel). This will blow the case out to fit the chamber so dies can be made. Due to the massive stretching(possibly in the web area) involved, these cases shouldn't be used for loading, only for making the dies. If the ase isn't formed perfectly, increase the bullseye until they are, one grain at a time. In the .17AH, I use 7 grs. bullseye and piece of toilet paper for the plug, so 10gr. should be just fine in the .303. Do NOT use a bullet for this forming.