I went on this mission when the 300RUM's came out, I went out to gun stores with my stoney point and a few different bullets. Now, in most it appeared the 3.65" was too tight in the mag. so I would back them down to 3.62" max COL.

Shortest chamber I found, was with a 200gr swift a-frame and that was loading to 3.75". Most hovered around 3.82". Thats almost .2" of freebore. Not near like a weatherby but alot of freebore.

I know a guy who had a custom 300RUM built and he built it so he could touch the rifling and still put it in a 3.6" mag. He couldn't use any loading data whatsoever that was out there. And never did really achieve any groundbreaking velocity. So the actual freebore is needed to achieve some velocity. He couldn't even shoot factory ammo. So think twice before messing around with that freebore.

I have three RUM's and none could you get that close to the rifling and still fit in mag. One was a custom built one with a krieger barrel and done with a clymer reamer.

I highly suggest to not put so much emphasis on touching the rifling if its a hunting rifle. It will cause more headaches than anything. If have the need to be .002" from the rifling for target, I suggest the heaviest bullet you can find. I tried it with 200gr matchkings and it didn't do anything groundbreaking.