6.5x55 any good for deer

Just so happens I learned that the hard way a couple weeks ago. I was out Mule Deer hunting with the .308 AR10 I built loaded with Hornady Superformance 150gr SST factory ammo when a 6x6 Elk presented itself. At 300yds all the shoulder shot did was make the poor fella limp. The 250yd hit through his spine ½ way between shoulder & head fortunately dropped him like a sack of potatoes. Had I had my 6.5x55 with the 140gr Berger VLD Hunter round I've developed, I have little doubt his heart would've been pulverized with the 1st shot.
One is a light for caliber short bullet. The other a heavy for caliber long bullet. There's a substantial difference in sectional density and penetration potential there. That 150 Sst isn't great on penetration on heavy game. It works fine for whitetails at sane velocities. If elk were in the cards I'd prefer a substantially heavier bullet in the 308.
 
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