What is the relationship between barrel length , caliber, and case capacity? I would assume a longer barrel with a larger bore diameter would allow more powder to be used. I would also assume that a short, small bore would require less case capacity to be as efficient. If that's true, what cartridges align to different categories better?
i.e.,
26" barrel .33 caliber = 33 Nosler or Lapua
26" barrel .30 caliber = 300 win mag
26" barrel 7mm caliber = 280rem
26" barrel 6.5mm caliber = 6.5 creedmoor
26" barrel 6mm caliber = 6x47 lapua
Barrel length needs to go up as powder volume goes up if a bullet stays the same mass and diameter. Also, barrel length needs to increase if the powder volume stays the same and the bullet diameter drops or the bullet mass increases. Any increase in need for length will come with a commensurate slowing of the burn rate for the optimal powder.
There's no formula that's going to help the layman. Suffice it to say, as you get more overbore (dead giveaway: there's a wide shoulder and/or longer case involved) you need more barrel. Overbore is basically the inefficient stuffing of more powder behind a slug to get a little velocity increase. It takes a huge amount of powder to get much velocity in most things after about 2500fps.
The reason for added barrel length and slowed powder is 2-fold. First, with too much powder in the case you need to slow the burn rate so it doesn't become a pipe bomb. If it all burns at once the chamber would open up and a bad day happens. Slower burn rate means more barrel volume needed to do it in because it's literally burning farther down the barrel.
There's a lot more to it but that should take care of the question at hand.
Thank you. For longer ranges is it better practice to use case capacity to increase velocity of the bullet or keep velocity ~2500 and increase projectile weight? I imagine that it depends, but for my level, I would assume rules of thumb are easier to start with - or not?
really depends because there are intersecting curves involved. The overly simplistic way to put it is, you find the point of diminishing returns with bullet BC and construction and then move up in case capacity until your needs are met. Best way to go for that via calculation is ping me direct at ballisticxlr@gmail.com and we can crunch oodles of numbers. If you're a graduate of a Rex Defense class held within the last year that's a free service I provide. If not, you'll need to get a support contract from ballisticxlr.com. It's very much worth it to race some paper before deciding to cut metal. It'll save you a lot in the end to be sure at the beginning.