Whoa, JVS!
I don't think any one would want to insult your grandfather's advice in any way; and like me, would implicitly respect and admire him because he was an elder and a hunter. In the context of his life, his knowledge, his means, and his experience, it was great advice!
I started hunting in the 50's, so I was around in the 60's. Deer and deer sized game have been stunningly put down with sub- .30 cal cartridges for a century (+/-). There is no argument that the .25's, 6.5's, .270's, 7's, etc. (some dating to the 1890's) are and were totally adequate for deer and much, much more. That is just a fact.
My first and favorite deer cartridges are .30's. Me and your grandfather would have gotten along just fine. But, you said that "I often wonder if my grandfather was right or if everybody just didn't get the word." So, I think it is quite respectful and fair to offer some information about other cartridges that are absolutely just as effective (if not more so) than our old favorites.
Back when your grandfather gave you that advice I would venture to say that the average fellow had a shotgun, a .22, and maybe a deer rifle. The "average" rifle buyer back then was more of a subsistence hunter, and equipment choices were more limited -especially in terms of cost, knowledge, and availability. "High-powered" rifles were more of an exotic consideration to some folks. The old arguments of bore size and bullet weight vs. velocity (Keith vs. O'Connor) were more heated and popular at the time. With ex-GI's from three wars, .30's ruled. There is a lot to be said about
context. Maybe your grandfather was trying to make a point with you, and felt that was the proper context to present it in.
I don't think there was any disrespect implied or offered to your grandfather or his advice. I'll bet he was a great guy and a good hunter.
