INTERESTING! I have listen to Erik on lands. He did note that as his groups open up he increase his length. He wouldn't set all his bullets to a set demension, by leaving them long and adjust as things changed.Hopefully a true expert like Alex Wheeler will respond, but it seems that with modern chamber designs and their tighter tolerances, getting a specific jump to the lands of the barrel is no longer seen as important as it once was. Erik Cortina, one of todays top competition shooters, made a YouTube video entitled: "Chasing the lands is STUPID! Don't to it." The current trend is to just find a combination that works well with a particular rifle/powder/bullet combination and not worry about the length of the jump.
The Weatherby Magnums (and most recent cartridges) use a tube of snug-fitting freebore before the tapered "forcing cone" into the rifling. Some cartridges like the 30-06 or 300 Win Mag just have a cone that tends to be larger at the mouth than the freebore sections for most modern cartridges. The 300 Win Mag's cone, for example starts at .315"dia. The 300 Weatherby mag, in comparison used a .361" long section of snug freebore tube ( 0.3084" dia) to allow the bullet to travel a little farther before hitting the lands and slowing down as the rifling got engraved. This resulted in higher velocity, but required pretty tight tolerances by 1940's standards. Unlike the larger cone of a 300 Win Mag, the tight tolerance on the freebore section of the Weatherby holds the bullet straight as the rifling is engraved. FWIW, the recent 6.5-300 Weatherby Magnum has about half the length of freebore as the 1940's ones, so it seems Weatherby has decided it had too much of a good thing.
The 25-06 is an interesting case, since when it was standardized by SAAMI in 1969, it had what was considered an unusually long and tight section of freebore ( 0.105" long with only 0.0005" clearance). It has the case body of the 30-06, but a freebore/throat design more related to the .257 Weatherby Magnum. The trend for the majority of cartridges released in the past 50 years has been to have a tube of freebore with a thousandth of an inch (or even half of that) clearance around the bullet. This holds the bullet in line with the center of the bore while the rifling is engraved. This had been done with "match chambers" for competition rifles long before it became common with hunting cartridges, so the term "match chamber design" is often used for the 25-06 and many newer cartridges. Today's better tooling and machining processes makes holding these tight tolerances much easier.
The older chamber designs with only an oversized cone for the throat rely on the case to align the bullet with the bore. For a number of reasons, this can allow the bullet to be angled slightly when the rifling is engraved on it. The result is that the rifling is not engraved evenly and the bullet will tend to be cocked very slightly sideways as it spins down the barrel. On some pictures of recovered bullets, it is possible to see this uneven depth of rifling on some of the grooves. When the bullet leaves the confines of the barrel, the forces from the rapid spinning of the misaligned bullet can cause it to jump slightly sideways.
That was one of the reason I started to purchase my reamers instead having the smith use his. The other is I want my neck area inside the chamber tighter than what factory builds them at.
The other is I was shooting a 308NM chamber rifle. Cases were starting to separate in about 3 firing. So I change out to a neck sizing die in a 300WM. It only sized about 2/3 to 3/4 of the neck in the 308NM. It stopped the case separation and improved the grouping. The case neck were cut for thickness so I feel the bullets were lined up with the lands.
It happens when you build good rifles.
