Tag Archives: seating depth

RELOADERS CORNER: Seating Depth Issues

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Don’t take anything for granted! Safety and suitability are both at risk if you don’t take time to analyze and act on this important topic. READ MORE

land illustration

Glen Zediker

As said often, it’s sometimes recent experience that leads to my Reloaders Corner topics. Whether it’s a question I’ve been asked, usually, or, in this case, a malfunction I’ve had, those things are fresh in my mind. I hope to believe, and have to believe, that any such topics aren’t only a question for them, or for me.

That brings us to bullet seating depths, which really means overall cartridge length, using some particular bullet.

Usually, when we’re loading for a rifle with a box magazine, either bolt-action or semi-auto, the cartridge overall length — that’s measured from the base of the case to the tip of the bullet — defines and determines the maximum length. Usually.

What ultimately determines the cartridge overall length maximum, though, is really the first point of contact that the bullet makes (will make) with the rifling or lands ahead of the chamber throat. That space, and therefore overall round length, has a whopping lot to do with the chamber reamer specs, and also the reamer operator’s judgment in some cases, but we need to know.

It also can have a whopping lot to do with the bullet! And that’s what the most of this next is all about.

So here’s the lesson to learn, and, for me, to relearn: Do not assume that if the round fits into the magazine it will be fine. I will, at the least, freely admit to my mistakes because, one, I dang sho should know better, and, two, if I know better and still don’t do better confession is my punishment. Well, not really, but it’s always a wake-up call.

Different bullets have different profiles, different ogive architectures. The ogive is the “curve” beyond the last point up the bullet that’s caliber diameter (meaning full diameter) ending at the bullet tip. My slang but descriptive term for this is “nosecone.” Tracing up this curve, some point will be equal to land diameter. So where this point is on the seated bullet and where this point is ahead of it in the chamber matters a lot.

Unless it’s done as a deliberate tactic, there needs to be some space, some distance between the land diameter point on the bullet nosecone and the lands. The amount of that distance is referred to as “jump,” because that’s descriptive. It’s the gap the bullet has to cross through to engage into the rifling. Usually the closer the better, and that “tactic” used often by precision shooters (mostly long-range and Benchrest competitors) is to purposely seat the bullet so it’s touching the lands. That’s done in the belief that if there’s no jump, then there’s no ill effects from jump. It’s very often right, and I’ve proven that to myself many a time. It’s not always right, but then if it was this all would be too easy.

The reason there needs to be some space is because when a bullet goes from just off to just on the lands, pressure jumps. It’s a “spike,” not a surge, but it’s enough to put a load that’s nearing the edge over the edge. In something like a .223 Rem. it’s about a half-grain-worth of propellant.

hornady 52
Here’s one I messed up with. The ogive or nosecone profile on this bullet is much “higher” than normal for a match bullet of this weight and it encountered the lands at a much shorter overall length than any others I had used. I learned the hard way, even though I already knew better.

So. Here’s the lesson I learned again, but this one wasn’t my fault! Honest! Several years ago, however, here’s one that was my fault: new (to me) match bullet, a short 52-gr. I wanted to try for reduced-course NRA High Power Rifle events. Rifle had a Wylde .223 Rem. chamber. A Wylde has a throat length between a 5.56 NATO and a SAAMI-spec. .223 Rem. That means the throat is fairly much more generous than commercial .223 Rem. specs. The maximum cartridge overall length in an AR15 box magazine is 2.260 inches, and I go 2.255 for a margin. I checked some industry manual data for this bullet and did notice that the overall cartridge length listed in the data spec table was a good deal shorter than that. I quickly did some “math” but without numbers (so it wasn’t really math) and decided that since I had a longer chamber I’d ignore that and just seat the bullets to 2.255. Blew primers right and left.

Back home and gage in hand and, dang, they weren’t kidding! I was about 0.020 into the lands at that cartridge length. That’s a honking lot. That’s also ultimately dangerous because of the free-floating firing pin tapping off the primer when a round is loaded into an AR15. A bullet that’s getting jammed into the lands is greatly more resistant to chambering freely and fully.

I humbly learned my lesson.

Get a gage and use it! The best out there is the Hornady LNL Overall Length gage. This tool lets you very easily find the overall round length that touches the lands with your bullet in your barrel. Very valuable, that.

lnl oal gages
A Hornady LNL OAL Gage will show right quick like and in a hurry with the seating depth that touches the lands is with your bullet in your gun. Valuable!

Use it in conjunction with its companion “bullet length comparator” insert for the very best precision. That tool measures a bullet at a point on its ogive that (usually) corresponds closely with land diameter. It won’t be perfectly the same, but it doesn’t have to be. What matters is that it gives a more accurate figure. Avoiding the bullet tip in a measurement eliminates that (guaranteed, by the way) inconsistency in accurate measurement because of bullet tip variations.

LNL comparator
A “comparator,” like this one from Hornady’s LNL line, is a much more accurate way to measure seating depth because the bullet tip doesn’t get involved. I like the curved one: easier and more accurate by my experience.

Now. To the recent experience: It was with a .300 Blackout (AAC) subsonic. I did not have the means to gauge this using my tools (then, but I do now). However, that wouldn’t have mattered in this case, and why is next.

Tested a factory load. Liked it. Noticed nothing unusual. Functioned perfectly, shot well. Brought it home and filled a magazine, loaded one in the chamber, and set it aside. Folks, just so you don’t think I’m irresponsible, that gun is what I keep at the ready for home-defense. So, my son, who had gone in to unload and then dry-fire the gun, came up and said, “Dad. The bolt won’t open.” Dang. It wouldn’t. I started thinking up all reasons that might be behind that. The bolt carrier would retract a little way, which was the limit of usual “play” in the bolt travel inside it, so I didn’t think anything was broken. To remove the round I pulled off the upper, took it to the shop, and pried back the bolt carrier from the underside. A couple of careful but firm enough strokes and it opened.

The bullet had really jammed into the lands! I mean really jammed. Extracting the round and looking at it, land impressions were clear, and measuring the extracted round showed it was 0.022 longer than the new, un-chambered round. Unseating the jammed round pulled the bullet that far out from the case neck.

I manually inserted another round of the same into the chamber and gave it a nudge-in with my finger, and, sure enough, there it sat not nearly fully into the chamber. Had to tap it back out.

jammed bullet
Here’s the “stuck” round, right, talked over in the article. Land impression is pretty clear, and pretty deep. Notice also that the bullet got pulled out a might upon finally opening the action. On left is the same round out of the same box that was pushed into the chamber; land marks also, just a lot lower!

So. Since it’s a factory load, I really couldn’t have had a clue that it wasn’t compatible with my chamber throat. But now I do. And, for a clue, do that same yourself. If the round won’t drop in and out of a chamber fully and easily, that might be a problem. I still don’t know what the actual measured amount of the excessive length might have been. To find that I’d have to get a box of those bullets and gauge them using the LNL tools. I’m not going to do that. I’ve chosen another load that’s no-issues.

I say “might be” because, again these rounds functioned well, but, also, well, that can’t be good…

I suppose I will now need to start handloading for that contraption. I have also written down 100 times: “I will always check the chamber throat, even if it’s not a long-range rifle…”

Find gages at Midsouth HERE and HERE

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