Not sure how much clearance various moderator makes give the bullets but I have used a .243 Wildcat P8 on the 6.5x55 with no problems. In fact I used the same moderator on the .243, 25-06 and 6.5x55. Checked a 0.277" bullet and it would not enter the baffle stack so concluded that 0.264" (6.5mm was the maximum). Before using the moderator on the 25-06 I spoke to the makers and they said it would be fine. I checked clearance on the 6.5mm bullet before deciding myself it would be OK.
Only ended up with the .243 Moderator due to a mix up in the shop the packaging tube said .308.