I don't know what you mean by a 264 shell. Are you looking at a 264 Win Mag shell? If so the casing would be bigger than a 270. The 270 bullet is actually .277 and the 264 bullet is truely a 264 diameter bullet The 260 Rem, 6.5x55 mauser and 6.5x284 are all 264 bullets. The 264 diameter bullet is also known as the 6.5 MM bullet. The 270 is just a necked down 30-06.