Just go by weight, everything else is the same, no big deal unless you decide to start loading up lead cast or a solid copper projectile.
UMMMMMM NO, not at max loads anyway. For most purposes though you are right on the money.
Boat tail bullets or ringed bullets don't have as much bearing surface against the barrel and that can make a difference in pressures. Whether it makes any difference in initial pressure spikes I'm not sure. Ganderite could likely tell you if there is a difference.
Last spring I was talking to a young fellow that was swaging his own bullets with boat tails and copper jackets with lead cores. He showed me a couple. Very nice looking bullets. Extremely aerodynamic but with a noticeable difference.
Each bullet had been chucked up in a collet on a lathe and had "rings" turned into them with a specially ground tool. The rings were very close together and about .005 in deep. Slightly more than most styles of modern rifling. The rings, covered most of the bearing surface of the bullet.
His reasoning was that by cutting the rings, he would be reducing friction between the bullet walls and barrel walls. I haven't seen him since. I would like to contact him again and see what his results were. He was expecting about a 10% increase in velocity without any increase in pressures.
I'm glad your thread reminded me of this. Now, I have to go into the shop tonight and see what I can come up with for a cutter. The collet to hold the bullet isn't a problem. The cutter may be a bit more difficult to come up with. The rings need to be concentric, so they can't be threaded.
Anyway, I'm sure you get the drift of this.
Bearing surfaces and twist rates can and do make a difference in pressures. At which point this actually makes a real difference has been discussed on other sites and no one with any real scientific knowledge or background has made any replies.
Something to think about though, on larger diameter artillery projectiles, they are equipped with driving bands instead of trying to imprint the rifling onto the projectile itself. This may be because of the extreme hardness of some projectiles in comparison to the cannon barrel. Not really sure. In such a case, friction would certainly be negated and likely pressures would be lower.