Tag Archives: bullet seating depth

RELOADERS CORNER: Bullet Seating Depth

Facebooktwittergoogle_pluspinterestyoutube

A popular topic in these pages, and for good reason: it can make a big difference in rifle accuracy! Read more about it HERE

benchrest bullets
Pretty much all bullets respond to seating depth changes. Long or short, for maximum accuracy it’s worth the effort.

Glen Zediker

Every time I do an article here in Reloaders Corner on the topic of bullet seating, I always see at least a couple of comments from readers about their experience and preferences with bullet seating depth. Those usually involve or revolve around seating a bullet so it is touching, or is nearly touching, the lands or rifling when the round is chambered.

This is a long-standing “trick” well known in precision shooting circles, like those competing in NRA Long Range or Benchrest.

Seeing What You’ve Got
First step, absolutely, is determining what the bullet seating figure is for your particular bullet in your particular chamber. This length is most often referred to as “dead length.” That’s a pretty ominous-sounding term! It’s not really perilous, but there is a little danger involved, which, mostly, is one point to respect. That point is that when a bullet goes from just off to just on — actually touching the lands — pressure will (not may) increase. Reason is that the previous gap-valve effect closed so burning gases are effectively “plugged up” a fractional millisecond longer. My experience with the most common small- to medium-capacity cases we’re using (ranging from, say, .223 Rem. to .308 Win.) is that this is worth about a half-grain (0.50-gr.) of propellant.

Finding It
Those who have read much in these pages have seen the Hornady LNL OAL tool. This is a well-designed appliance that will show you, in your chamber with your bullet, how far forward the lands are, or, more precisely, the overall cartridge length that will touch the lands. This amount varies and is unique! Don’t transfer figures from one gun to the next. It also changes… As the chamber throat erodes it lengthens, and so too will the overall cartridge length that touches the lands. Let’s call overall cartridge overall length COAL for sake of space.

hornady lnl gage
Here’s the tool to find the seating depth that touches the lands. Hornady LNL Oal Gage.

There are other means but I’ve not found one more accurate. Some smoke over a bullet that’s been seated into a “loosened” case neck and gauge contact by the marks left. This, however, is likely to be “touching, plus” length.

Once you’ve got the round ready to measure, I strongly suggest doing so using a bullet length comparator along with your caliper. This is another tool that’s been gone over and gone on about here. It measures at a point along the bullet ogive rather than on the bullet tip. It’s more accurate. Now. A comparator inside diameter is usually close to actual land diameter, but, as with chambers, these are each and both unique so don’t assume anything.

Hornady comparator
More precise reads come from using a bullet length comparator to measure overall length. This is a Hornady LNL too.

Why It Works
Setting the bullet so it touches the lands does a few things, all good. One, and I think one of the most influential, is that the bullet starts off aligned with the rifle bore. As a matter of fact, it better centers the whole cartridge because there is, not may be, at least a little gap between chamber and case. If there wasn’t the round wouldn’t enter the chamber. The bullet is, effectively, supported by the lands and that has, also effectively, taken up the “slack” by locating the cartridge more concentric with the chamber and bore. It also then effectively makes up for the affronts to concentricity created by case neck wall inconsistencies and the resultant relocation of the case neck center.

Another is that that it eliminates jump (the usual distance or gap between the first point of land diameter on the bullet nosecone and the lands). Bullet wizard Bill Davis (designer of the original “VLD” projectiles, and others of much significance) once told me that his thoughts on why especially the high-caliber-ogive high-ballistic-coefficient bullet designs worked best with no jump were for all those reasons and improvements just mentioned. Plus another: gravity. A bullet floating in space, and also moving forward in this space, has that much more opportunity to engage the lands at a little angle, if only because of gravity. Always have thought about that one.

Soft-Seating
There are degrees. When we go from just on to “in” that’s another tactic some experiment with. And it has another level that’s commonly popular with Benchrest and other precision shooters. That’s called “soft seating.” What that is, is setting the case neck inside diameter to very nearly match the bullet diameter with the idea that the bullet starts out extra-long and then chambering the round finishes the bullet seating when the bullet contacts the lands. The reason for the more generous case neck inside diameter is to reduce resistance so the bullet can more easily set back and let the lands seat it.

I don’t use this tactic, but have. It’s another level of commitment and, as is often true with such other levels, demands more attention and also limits utility. One is that it clearly is only for bolt-action use. Another is that it’s for single-shot use only; such rounds should not be loaded into a magazine or fed from a magazine. For another, once loaded the round can’t usually come back out. The bullet will stay and you’ll get an action full of propellant.

Seating Depth Experiments
Now this is a process I have used throughout. Most times I find that best accuracy comes with a seating depth that has the bullet “just” on the lands. Contact is made but it’s the same pressure level as if the bullet were sitting on the benchtop. I also often have found best group sizes come at a little less than touching, and, a few times, at a little more than touching. I’m talking about 0.002-0.003 longer than dead-length. Let’s call it “firmly touching” but also a long ways away from “jammed.” These rounds often can’t be extracted.

There’s an easy way to run seating depth experiments. Here’s how I do it: I load however-many rounds at dead-length plus 0.003 COAL. I load them all that way. I then take a small press I can clamp on to a benchtop or tailgate at the range, and install a micrometer-top seating die. For max accuracy, I already seated all these test rounds using this exact setup. Take along a caliper and comparator and a fresh notebook page. I’ve adjusted the propellant charge as said earlier by dropping it a tad. Now. I also know that there’s going to be a little difference in perfected results because of this because lengths that aren’t touching the lands are running 35-40 feet per second slower, but it still shows me what’s going to work best. If it ends up being a COAL with a little gap, I’ll bump it back up.

Last
As said, the COAL that works best is going to change because the throat is going to change. Check using the OAL gage and adjust. That means the load is also changing, a little bit, each time the bullet moves forward (more case volume), and that can affect zero and velocity.

It’s a lot to keep up with.

Another note: If you’re feeding these rounds from a magazine, and running them through a semi-auto match-rifle, make sure there is adequate bullet retention (difference between bullet diameter and case neck inside diameter, go good 0.003 inches). Don’t want the bullets jumping forward (inertia-induced). If, for example, you’re giving 0.002 hold-off, that little bit can get taken up easily and then, if the bullet gets on the lands, there’s a pressure spike.

GAGES, on sale now at Midsouth!

The preceding is a specially-adapted excerpt from Glen’s book Handloading For Competition. Available HERE at Midsouth Shooters Supply. Visit ZedikerPublishing.com for more information on the book itself, and also free article downloads.