Re: 300 RUM accurate hunting load
I'm .180" off the lands at 3.660" OAL, and yep, that's part of it to be sure. I'd guess I'd have to drop a grain or two if I run up on the lands with them or I'd run up closer to 70,000 psi, which is usually about my max anyway.
If you're at 3000 fps, i'd suspect pressure to be nearly where I am, just might be your barrel is a bit tighter than some are and you get by with a might less powder.
With Retumbo, I started to see some ejector impressions early on, where RL25 did not produce any until I was up over 70,000 psi and nearly matching the MV with Retumbo. Retumbo seems to batter the brass a bit more, but at lower pressure for some unknown reason. If I went to 70k psi with Retumbo I'd get looser primer pockets, I could already feel them coming at 65k psi on some of the test loads, which was a little unusual, even with the softer Remington brass.
Take a look at the Barnes manual, I was at 120gr RL22 on the first lot# with a 400gr X bullet in my 416 WBY before I reached 2700 fps, which was 12gr over max load...
Working up from 104gr with each lot# just to be sure, I found lot to lot consistancy to be between 117 - 120gr in that load. I was so curious to see if my rifle was just not capable of that speed at max load that I broke down and bough a box of factory loaded Swift A-Frames ($99.00) to check the speed on them. I got 2650-2680 out of them and was quite relieved. Heck, my barrel was only 23.5" long in this Ruger verses the 26" bbl's I'm sure Weatherby uses to establish the 2700 fps with a 400gr. Since then I've been using the Oehler M43 and found more such good news about my rifle, like 2700 fps is about max, but still asily doable at 65k psi, even with RL22. I can't wait to try RL25 and Retumbo in it.
MV is one of your best indications of top pressures, that and bolt rotation and extraction ease goes a long way.