For elk I like magnum 30’s, although I have shot 3 bulls with 30-06 and ‘gasp’ 180gr ballistic tips, one with a 7 WSM and 140 AB (deer load but did the trick) and 2 with the 300 WSM and 168 TTSX.  The 300 ultra and 200 AB’s was awesome on 2 bulls but is a heavier rifle. 
  As to why shoot a magnum, as an out of stater with limited time, I want to take advantage of ANY reasonable opportunity, and a 300 mag offers just that.  From 50 to 500+, even 600 they offer greater impact, penetration, and visible killing power. 
  I built my ‘ideal’ carry rifle several years ago with a 26” #4 Brux straight fluted, TI action, ADL bottom and a classic McMillan sporter stock (with a 1” Decelerator).  With scope it’s just a hair over 7lbs and handles perfectly with the 2.5-10x50 Leica ER scope with a simple ballistic reticle.  Just me and my idea of a hunting rifle.  It’s a bit much in the prone position or range sessions but in the field does everything I want.