Ah, riveting. That can be a very real thing.
Ian had an excellent example in one of his rifles. He found little lead rings at the end of the case neck. Think of what happens to our relatively putty like bullet when you hit it with 30 K PSI. If I recall correctly he was using a bullet that was shaped to fit the leade angle exactly. So, the nose can't go anywhere but the base is suddenly hit with a bunch of pressure. What happened? The bullet expanded, or riveted, and some was extruded into the little space between the end of the case and the end of the neck portion of the chamber.
Riveting can be of benefit, it is essentially what makes many ML projectiles work. Think of a minie ball in a rifled musket. Undersized until ignition, the pressure causes the projectile to expand and fill the bore.
Don't forget that alloy and load make a difference in riveting too. Sometimes it isn't a bad thing as it can allow a nose to fill the rifling a bit better.
More nose taper, beginning with a smaller diameter, isn't a bad thing. A bullet that can fit as many rifles, or as many worn throats, as possible is a good thing.
Leaving the neck flare is a tool I know fiver likes to use. I have done the same Ina few rifles to help center a bullet then neck thickness wasn't near enough to fill a generous chamber.
You are looking to learn and asking questions. That alone makes you more likely to do well than most who simply want to be spoon fed. In many ways that is exactly why this forum exists. We wanted a place where we could share info. We may not always agree but we can disagree in a polite manner. There also isn't always a single right answer.
