Ohhhhh, but there's so much more to it. Remember my bullet design for the XCB? The one Brad and I both got back when the project first took off? Remember me going on about doping out the "Morse taper fit" design? Remember why it was a failure?
This is for Lewis, I'll try to explain static and dynamic fit as I understand it here, as a sort of rough draft for a future, illustrated article. I explain by example and you have to use your imagination to form a picture of my descriptions.
If you do make a positive, matching, taper fit design, have less than a thousandth of total loaded neck clearance to the chamber neck, and the jam the bullet hard against the ball seat to force perfect alignment, then three things happen, all of of them bad, one resulting from the other: First, the engraving pressure is extremely high, so the powder pressure builds very quickly and to a very high pressure before the bullet even begins to move. This is like an air compressor without an unloader valve having to start from a standstill against full head pressure, or a locomotive trying to start a train of cars all at once without having any slack in the couplings, IOW VERY high load to overcome static inertia and forward resistance. Second, that very high load is applied where? The base of the bullet, right? By what means? Gas pressure, right? So if the pressure to move that bullet is quite high, the neck is going to obturate the chamber before the bullet gets a chance to move and gas pressure rushes around the gas check and flame-cuts the hell out of the back two driving bands of the bullet. Then, as the bullet starts to move, the base gets wadded up like a soda straw wrapper. None of that translates to sub-moa groups. Third, any metal smooshed up by the pressure behind bulges the base and then gets raked off and swaged into the only place it has to go, which is the little space in between the case mouth and end of the chamber neck. I've pulled a lot of lead rings out of there with a bronze brush when working with jammed, morse-taper bullets that weren't made hard as coffin nails, i.e. straight linotype. Anything soft or even sorta kinda fast powder, without a buffer in front of it, made hash instead of groups for me. A good example of this sort of fit would be the NOE XCB bullet and a freshly cut, SAAMI-spec .30-'06 chamber. To make it work you need a slow powder and not too tough of an alloy, but tough enough. Lots of tin to stiffen the bullet in all directions but improve flow. Another option is water-dropped 60/40 wheelweights and soft scrap lead, this should be around 2% antimony and less than 1/4% tin. Heat treated to 19 bhn this alloy will draw well, engrave well, but also be stiff and resist gross bending forces at launch. Hard, brittle alloys are a NO GO with a jammed, fully-matching fit. The CBA crowd almost all do it this way but they use a very gradual taper and the manage to get away with it.....within the inherent velocity limitations of Linotype alloy (more on that later).
Now, consider some other fitment options. The .308 throat and the NOE version of the XCB with tapered nose meant to perfectly match a 1.5° per side throat of the .30 XCB wildcat chamber or any standard .30-'06 throat. The .308 throat has a .310-.311" parallel freebore for at least .100" and an abrupt taper to the tops of the lands, making it in effect a three-diameter system if you count the case neck. Neck diameter, abrupt taper to freebore at the entrance, abrupt taper to the bore diameter, with groove diameter falling in between freebore diameter and bore diameter. Step, step, step. Not a good matching fit we would think. Got it? Ok, now you take a bullet like the XCB whose body is sized to fit that freebore, or middle step, and the nose is tapered so it can't really fit more than one point in the throat (which will be the front edge of the ball seat where it intersects the tops of the lands, assuming you jam it in that far), and has no excess nose hanging out front dependent on the tops of the narrow lands for support. Got that? Only a tiny bit of the bullet is supported by freebore, the rest is supported by the case neck in the chamber, and that neck support is going to go away at a certain pressure level. That pressure level has to be less than engraving pressure. The ONLY reasons this works is due to the throat mismatch and the bullet being engraved gradually and not all at once, and as the dynamic change is happening to the bullet, remaining straight in line with the bore as it is drawn through the throat actually IS the path of least resistance. The force to move the bullet from rest is slow, the increase in resistance is gradual, and the pressure doesn't peak to bullet-damaging levels until the bullet is safely moved straight into the throat (assuming you used a suitable powder burn speed to make that happen). Make sense? This scenario working out well is also dependent on a bullet alloy which is malleable enough to deform without too much resistance, but still tough enough to guide itself straight with very little support. It's a balance. Change one thing out of the functional range and it goes to hell. Make the alloy too soft and it rivets in the case neck even though the engraving pressure is low. Use too fast of a powder and even a tough bullet rivets and gets washed out base bands before it is able to transition to the safety of the throat. Screw up your static alignment with an off-center case neck and it has no chance.
Take that same .308 bullet launcher and try to match the hatch to those three steps with abrupt transitions. It works if you don't depend on the nose for alignment or have too much nose out front. Your launch pressure will be extremely high if jammed because the bullet has to deform in several places all at once before it can move. If you use a tough alloy and don't have much bullet in the case neck, it can work pretty well, maybe, but things have to be PERFECT.
Now, let's look at the reverse fitment scenario, which doesn't work for beans above about 1800 fps but is pretty much THE world standard and why almost everyone fails at high velocity: A throat with a straight taper, or one which has worn from use into the typical parabolic trumpet-bell shape, and poke a bullet shaped like two, different diameter cylinders into it. The nose cylinder is sized for a slight interference fit for best results (ask anyone), the back half of the nose in front of the front driving band supported only by air, the front edge of the front band is supported at only one point somewhere in that throat taper where the sizes match, and the majority of the bullet body is supported only by the case neck which most of the time is so crooked or off-center in the chamber due to standard, jax loading dies that the bullet has no hope of alignment. SO in review, you have the tops of the lands supporting half the nose (figure that surface area out, it's infinitismal), one ring of contact with the front edge of the front band (also infinitismal and theoretically zero, even if the band is jammed into the throat taper) and the case neck supporting the rest. Basically you have a static fit supported only by hopes and dreams, and the thing that's doing 99% of the support work (the case neck) is a house of cards. Light the primer and the bullet goes any way but straight as it moves into the bore, it gets mangled, engaved more on one side than the other, and comes out the muzzle wobbling like me throwing a football. That wobbling causes massive group dispersion, and the faster you go the worse it is because the bullet damage gets worse as launch pressure necessary for higher velocity is increased, and the external ballistics play more and more on those imbalances the faster the bullet spins. This is what was happening to those 311299s at 2300+ fps. If you recover some from that snow bank in the spring, you'll see the effects of what I'm talking about.
Now, let's talk about yet another scenario: How to choose a bullet to fit a typical, worn throat shape or either of the above conical or stepped throats. If the bullet has a concave nose, nothing parallel except the driving bands, plenty of displacement area for lead to move so that launch pressure is low and the bullet base doesn't see damaging forces until sealed up and safely engraved straight in the barrel, then the exact points of fit and support will come naturally to the bullet both statically and dynamically. The key to making it work is a weak alloy but heat treated or powder coated for toughness and abrasion resistance. These bullets can be loaded with medium to slow rifle powder and jumped a considerable distance and still shoot well. That means ammunition can be made to fit a variety of rifles with subtle differences and interchange with acceptable accuracy. That means that the ammunition will function in a dirty rifle or cold rifle or if the bullets grow a little over time they will still fit and function. Not a picky thing. Also, the ammunition can work in an autoloader with weak camming action because you don't have to jam the bullet. Even a suppressed autoloader that cakes up the chamber with filth and grime. You don't have to fit your cases to the chamber as tightly as with the other fitment methods because the bullet can self-align (to a point) and the STATIC fit of the cartridge in the chamber isn't nearly as critical. This is a highly dynamic system, very versatile, and very accurate if done correctly. The drawback? Yes, there is one. There are VERY few moulds of this self-aligning design. The best ones are offered by MP Molds, designed by Bob Kell (45 2.1). The .22 NATO, 311-180 Silhouette, and 359-220 are perfect examples of this design. If you want drawings of similar concepts (Bob won't release a drawing to the public), look at Accurate Mold's catalog for the 31-188G and 31-157G.
A word on the velocity limitations of linotype and other hard, brittle alloys. Unless you give them a jacket (paint, paper, copper), they begin to fail at high velocity because of land stresses on the sides of the engraves. The alloy crushes on a microscopic level under stress and falls apart. The engraves get washed out a little and the gas jetting up the engraves when the bullet exits the muzzle crown throws the shot, and then the next, and so on but rarely in the same place. Your bullets to not have to be rock-hard and brittle to shoot well at HV, quite the opposite in fact. Again, alloy has to be balanced to the system like everything else does because the whole system has to work together to work at all. Change one thing, you'll likely need to change something else to restore the balance. Some systems of dynamic fit require different powders, neck tension, jump, primers than others, possibly a buffer, probably a very specific alloy, and so on. Some alloys handle fast rifling twist rates better than others. Some bullet designs prefer wide lands, some don't care. You likely won't have good results if you just grab a bullet that looks good and stuff it in any old rifle without regard to dynamic fit or how to set things up to optimize those dynamics for a balanced, undamaged, straight-shooting bullet to emerge from the muzzle.
Fit. Alloy. Powder. The big three factors for high-velocity cast bullet accuracy. Understand how to choose wisely for your desired application and you will have success.