This is an old urban legend. Elmer Keith once wrote in his book " Keith's Rifles for Large Game" the .375 270 gr bullets being tougher and better penetrators than the 300 grainers. Himself, he repeated that sentence in other articles, f.i. in "Guns & Ammo". Others took these words of the old master as gospel and recited them again and again, without really checking or testing. Keith's book was first published in 1946. Hornady bullets did not even exist then. Keith wrote this, based on limited eperience, about the only pre-WW2 made in USA factory loads of the .375 H&H Magnum, offered then by Winchester - Western. Many others simply copied and pasted the verdict without checking the background. So everyone assumed the 270 grainers, regardless of make and construction, being tougher than the 300 grainers.