Well, everyone's experiences differ. My late friend, Don, who bought that .62 rifle, had been shooting and "fixin'" flintlocks all his life. He was a fine shot, off-hand or off a rest. He was one of the most dogmatic supporters of the "rule" about choice of powder vs. caliber, that I Knew then, or now. He almost went into shock when he fired his targets with 3Fg and got a much better group off a rest. He showed everyone-- and I Do mean Everyone - both his targets, and asked everyone if they had ever experienced anything like it.
It was months later when the first chronograph showed up at the club, and he had a chance to revisit his powder choices, trying groups off the bench with both 2Fg and 3Fg. He even had other good shooters in the club shoot his gun, with the different powders, and the group size, and the SDV all pointed to 3Fg giving better accuracy for him. His patches told him the same thing. We all looked- to be sure.
Since that happened, I have run into several people with guns that don't fit the rule, one way or another. I was personally surprised that 2Fg would give better groups in a .40 caliber rifle, when it happened to another friend. I had been WILLING to accept the idea that 3Fg might just work well in calibers larger than .45-.50. I just didn't want to believe that the reverse was true- that 2Fg would work better in some smaller caliber guns. Before I met Don, I had a friend who used 3Fg in his .54 caliber rifle, with great accuracy. He was the man to beat at the club matches, before Don joined the club.
I also knew older friends who were shooting 3Fg powder in their .58 cal. rifles, but that was before I got into the sport, and even heard about "The Rule". I didn't know that what they were using was "wrong".
This was all before chronographs became portable, and less expensive, so that ordinary shooters could afford to own them. We read spent patches, and compared group sizes to decide questions about components, not much different that what Dutch Schoultz teaches. Because of my experiences, I used to caution Don about his Dogma, and that would cause strong arguments in favor of the " rule". Since I didn't own any such gun- I was shooting a .45 at the time--- There was no way I could demonstrate that the Rule might not work all the time. Don had to prove it to himself, He did feel a bit sheepish about his having questioned my ancestry in some of his arguments with me about keeping an open mind. I am only glad I am NOT the man who suggested he try 3Fg powder in the gun. He might never have gotten that gun to shoot right!
[ Oh, and you can be sure he confronted me, and asked me if I was going to say, " I told you so!" I told him absolutely No way would I say that. I didn't know anything about the rifle he was shooting, nor anything about other .62 cal. rifles shooting PRB. I had never seen such a gun in that caliber shot, anywhere. ]
So, Spence, let me keep my open mind about barrels. I may be wrong, too, but I am comfortable that I won't be caught by surprise when I suggest someone try both powders and see which the barrel likes. :hatsoff: