Re: 270-300 win mag
Interesting....... I can't help much on loading data. You may wish to look at 7mm-300 Win Mag, I haven't looked but there is probably several of the out there, starting loads and back down from there.
I shoot an improved 300 RUM necked down to 270 out of a 3 groove Lilja 30" with something like a 7 twist, I think. Is made for heavier bullets. In the 160 to 195 grain range.
I have to be careful because of case capacity, shoulder angle and caliber that the powder doesn't bridge. Thus for the heavier bullets, 150 grains and up only ball powder is used. US-869 being preferred.
With the 140 Nosler Accubonds 89 grains of IMR 7828 puts it well over 3600 MV with great accuracy. I don't shot any of the Barnes as I haven't been able to get the accuracy of the sepciality or Nosler bullets. Bergers, because they seem to be designed for less than 3300 fps haven't worked out either.
Sorry, I couldn't be of more help.
Have fun and let us know how she goes.