The following is a specially adapted excerpt from the forthcoming book Top-Grade Ammo by Glen Zediker and Zediker Publishing. Now, it’s just around the corner… BuyZedikerBooks.com for details.
by Glen Zediker
Last time I showed a couple of valuable tools: the Hornady LNL O.A.L. Gauge and the LNL Comparator. The first showed the bullet seating depth that touches the lands, and the other gave a more accurate way to measure. Now for a few ideas on how to use this newfound information.
“Dead-length” sounds pretty ominous, and it is, or can be. Again: that is the cartridge overall length that has a bullet touching the lands when the round is chambered.
This tactic is well (very well) proven to promote accuracy. Usually. The overall idea is to introduce a bullet dead straight into the bore. Entry will be smoother. This is also an asset to concentricity, or centeredness. If not touching the lands, scooting a bullet out to be very near the lands helps, usually, and it’s a near-requirement for some bullets. The reason is pretty clear: the shorter distance the bullet has to “jump” before it engages the lands and enters the bore, the less can go wrong. Off-centeredness may then be less a factor over a shorter distance. The bullet is already in-line with the bore, and that overcomes any small alignment imperfections that might exist elsewhere in the round. Gravity also factors in, even in that short gap.
Dead-length seating is not the same as “soft-seating.” You’ll encounter that term. Soft-seating is sizing the case neck inside diameter to just barely less than the bullet diameter, and the idea is to have the bullet “out” and then let contact with the lands finish the seating the bullet into the case neck as the bolt closes. That requires very light case neck “tension” (less than 0.001 difference). Bolt-actions only! This eliminates the effects of any bullet dimensional issues. I’m not recommending this, but there it is. Pressure cautions are increased with this tactic! It’s common for soft-seating to engage into the lands by 0.010-0.020. So now you know…
Generally, the longer and spikier a bullet is (secant-style ogive, high-caliber-radius), the more likely it will be to perform best starting nearer the lands. For good example, although it’s contrary to some others’ experience, I have yet to get good accuracy from any of the true “VLD” (Very Low Drag) bullets until they get on the lands. Same goes with others similar, like the Hornady A-Max; those are more tolerant of jump, but don’t like much of it.
One — first, foremost, always — this tactic will raise pressure! There’s nothing to worry about here until the bullet actually gets on the lands. Going from “near” to “on” is a huge difference! The reason is the loss of space for burning gases to occupy, release been delayed because the bullet is working as a plug from the get-go. Reduce any load one full grain! Work it up from there. It might end up being more like a half-grain, but it will at the very least be that much difference. That’s significant.
Two is that approaching dead-length isn’t usually possible on any but single-loaded rounds (not fed from a magazine). This also goes for bolt-actions (and, honestly, I really only recommend this tactic for bolt-guns). Just depends on the space available for the rounds at rest: magazine length. For my ARs I do it only with “long” bullets used for the “slow-fire” portions of a rifle tournament (rounds must be fed singly). (With one exception, shown nearby…)
Three is that it’s going to change. As the rifle throat erodes it advances, distance to the lands gets longer, so what was “touching” some-hundred rounds ago is no longer “touching” now. Certainly, that varies with the cartridge, and will lengthen faster with a .243 Win. than it will with a .308 Win. Get the OAL tool, keep up with it, and it’s easy. However! The load is also changing, a little bit, each time the bullet moves forward, and that can influence velocity and zero.
It’s a lot to keep up with.
Make sure there is adequate bullet retention (diameter difference between bullet diameter and case neck inside diameter, go a good 0.003 inches). Can’t have the bullets jumping forward (inertia-induced). If, for example, you’re loading to give 0.002 hold-off, that little gap can get taken up easily and then, if the bullet gets on the lands, there’s a pressure spike. Goodbye primer!