Yup, it's definately the cartridge in the chamber that's rattling! You can hear it from the muzzle....I seated one of the 500gr bullets in a case to an OAL 2.55" with no primer or powder so I could trickle in powder to see how much it would hold, before I found out they wouldn't work at speeds I was willing to shoot in it.....it's definately loose in the chamber... :? I'll do a little micrometer measuring to see what the diffence is and where.
Sure is a big hole down that bore, makes me feel good having it in my hands, kinda like a muzzleloader in that respect, being a hammer gun with a big bore!