The only Remington 303 I've used was the CoreLokt ammo. The accuracy is great, but the fat projectile will not feed from the magazine.
As far as Rem brass goes, If the load is as powerful as the CoreLokt, it will bulge the brass and make it not very reloadable. I've read opinions about Rem brass and they were all good, but this was for brass purchased for reloading, not brass from cartridges loaded by Remington. I once purchased 100 rounds of Rem for reloading. Some of the first 20 shot with factory loads were bulged to the point where I was afraid to reuse the brass.
My conclusion is that if you want Remington brass - buy the brass. If you want the accurate cartridges and don't care about reloading - get the ammo, I'm sure Remington knows how to make a good cartridge.
If you want to shoot ammo and than reload... buy Prvi Partizan....