Just started reloading the .300 RUM with Redding die set #80288. Full length sizer adjusted to allow bolt to close with no resistance. Noticed the expander ball had very little resistance when sizing. Measured expander at .306 dia. When loading nosler and barnes bullets, bullets are loose. What should be the sized mouth dia measured with the expander removed from the die? I am using new Remington brass. I believe I may have an oversize die. Any comments will be appreciated.

The inside of the neck should be .002 to .004 smaller than the bullet diameter. As they will spring back just a little after he expander goes through them, the .306 diameter sounds about right.

Measure the OD of: a fired case neck, a sized but not expanded (remove the expander ball) and one that's been sized and expanded.

If you had just annealed those case mouths, I'd say that you overheated them during the annealing process (don't ask how I know about this)

Should have added this is the first reload on 20 rds of once fired remington factory brass and 10 rds of once fired new factory brass. Just measured new bagged brass at: ID @ .305-.306, OD @ .335 (plus half a thousandth) I,m woundering if the sizer die is oversized and not bringing the neck small enough. Can not measure fired brass as I just relaoded them this morning and will single load them and shoot at 100 yards. I plan on annealing the brass after 4 reloads, shoot three more times and scrap the brass. This seems to work well with other high intensity cartridge brass.

I had the exact same problem with the same die as you. I switched to Nosler Accubonds and problem was solved. I started out shooting TSX's and noticed the loose bullets. Noslers hold fine for me with the .306 expander.