Remember that .30 Carbine load I mentioned? With all the reloading stuff being put out right now, herein I will lay out how I worked that load up.
The general rule is that when switching to a new powder or bullet, and sometimes brass, you start low and work your way up. Powders have different burning rates, bullets have more or less weight and/or bearing surface and, depending on composition, can be harder or softer, and cases can have thicker or thinner walls(among other things). All of which means that changing any of those factors can make pressure go up or down, and can affect accuracy. And things that affect pressure can cause anything from less recoil to more to a semi-auto not functioning correctly to something failing. Sometimes catastrophically.
For instance, I’ve got a Turk Mauser in 8x57mm that I modified as a hunting/target rifle. Very solid, very strong and capable of good accuracy. While I was working up loads for it, I discovered that in 170-grain factory loads Federal shot very well; Remington just as well but hit about an inch higher at 100 yards; Winchester gave much worse groups that hit higher and to the right. I saved the brass to work up some 150-grain bullet loads. I hit a charge of IMR4064 that shot very well in the Federal cases with a Hornady bullet. The Remington cases gave equal accuracy with the same load, but- same as the factory loads- hit about an inch higher than the Federal. And the Winchester cases gave lousy groups that hit high and right. There’s nothing wrong with the quality of Winchester brass that I know of: that rifle just doesn’t like it. You run into that sometimes.
With the Carbine I was using the same brass, Lake City GI, and the same powder and primer but switching bullets. The Sierra Varminter weighs the same as the GI ball- 110 grains- but has a different shape and length. Measuring lengths showed that the Sierra would have to be seated deeper than the ball so as to keep the overall length within the maximum*. Being seated deeper means less space for powder. Some loads in some cartridges will compress the powder somewhat, and with some it causes them to burn more efficiently. In others it can cause dangerous pressure spikes. Which are Bad Things, which I would like to avoid. So I did some measuring. The bearing surface of the two bullets- the area where it actually bears on the bore surface- looked to be about the same length. I had no way to measure the comparative hardness of the jackets, so that was guesswork. Which I really don’t like, but I couldn’t find any ‘official’ information on this. It looked like dropping the powder charge one grain from what I’d used with ball for full-power loads would place the bullet base and the top of the powder column in the same relative position as the full charge to ball, so I decided to give that a try. So I loaded ten rounds and tried them next time I went to the range.
No chronograph as it turned out due to the dead battery, so I fired five on a 50 yard target. Accuracy** and where they hit on the target were same as ball, and the cases showed no signs of excessive pressure. The action cycled normally, and the empties flew out at the same angle and distance as ball. So I moved to 100 yards and tried the other five. Again the bullets hit about identically to ball and the other things were stable as well. So next time I set up the press I loaded ten more and made damn sure I had a good battery in the chrono. As noted a few days ago, velocity was just a touch above the nominal velocity for ball, and that combined with the other factors tells me pressure should be just about right for full-power loads.
No, this is not quite how they do it at the factory, or by the people who work up loads for a living. They’ve got universal receivers into which they can mount different barrels for different cartridges and/or strain gauges to indicate pressures, and a setup called a copper crusher(it uses a slug of copper of exact size and shape and alloy, how much it’s deformed tells you the pressure of the shot) that fits on barrels in the universal receiver; now there are electronic setups that do the same but without the copper being crushed. I don’t have any of those, so it was check around for loads others have tried, compare different factors, decide what should be workable and try it. Be very aware that when you step outside what’s in the manuals, you’re on your own. Screw it up(wrong powder, inappropriate bullet, too much or- in some odd cases- too little powder) and you can have a slight problem. As in ‘boom’. Resulting in anything from slight damage to failure of the action, anything from “Damn, that was loud!” to “Oh my, I think I’m bleeding.” No kidding, do not screw around with loads outside the manual unless you have the experience or equipment to deal with it. And experience doesn’t always count for crap.
*OAL, overall length, is important for a couple of reasons. One is simply that if a cartridge is too long it won’t feed through the action. Not a big factor with single-shot firearms, big deal with repeaters. Second is that there should generally be a short distance between the bullet position when chambered and where the rifling begins(called ‘freebore’ if I remember right). If you don’t have that space, it can cause the bullet to be delayed in leaving the case, which can cause a really bad pressure spike: Bad Thing. Third thing, if a bullet is long enough and seated far enough out it can hit the rifling and keep the action from closing and locking completely. VERY bad thing. On a bolt or a single-shot it will generally let you know there’s a problem, either the action will not close completely or is very difficult to close, both warning signs. On a semi-auto, if this happens and you don’t notice it and pull the trigger you have an ‘out of battery’ fire. Also known as a ‘kaboom’. It generally means a destroyed action and often means some level of injury to you.
**Accuracy in this case doesn’t mean ‘in the bullseye’, it refers to how tightly the bullets group. Different components in a load can cause the point of impact to vary, sometimes by a fair amount. That’s not a problem in testing, because when you find a load that’s good(tight groups, reliable, within specs for pressure and so forth) you adjust the sights to place it exactly.