Frankly, I could not disagree with you more. We have plenty of empirical data backing up the deformation of exposed lead bullets. What we have been treated to, endlessly, is the fast and light vs. slow and heavy argument, which you rehash here.
The 303 was loaded with a jacketed, not a cast bullet, and the cartridge was associated with so much fantasy, that like the 22 HP, it required a few hunters getting stomped into the dirt, and failing spectacularly, before reality hit.
You do, however, make a point that is correct-some loads simply shoot well in our gun, and we don't always know why. Probably this is the reason so many of us say our rifle "likes" a particular load.
I should mention that it was somewhat common in decades past to see handloaders go with a 180-190 cast for the 30WCF. Back then they thought these loads gave them an edge. The reason almost no one wastes their time here anymore is because today's premium bullets actually do provide an edge, without the loss in velocity or rise in pressure that comes with the heavy for caliber bullet.
