Will a 6.5 x 55 reamer cleanup a 260 Remington chamber?

I don't think so; although the 6.5x55 starts out 0.006" larger, it tapers ~0.041" over 1.5". The .260 tapers ~0.015 over 1.25".

Clarence

Nope The 55 & 57 based cases have to much taper to clean up a 308, 260 & 243 based chamber

Thanks!