Bullet Seating Depth Determination

Facebooktwittergoogle_plusredditpinterestlinkedintumblrmail
Facebooktwittergoogle_pluspinterestyoutube

The following is a specially-adapted excerpt from the book Top Grade Ammo, by Glen Zediker. Visit ZedikerPublishing.com or BuyZedikerBooks.com for more.

by Glen Zediker

Since I’m focusing on bullets for the next few times, I’ll say some more about the relationships of bullets and barrels.

Barrels have two diameters: groove and land. The groove diameter is the caliber size (within, usually, 0.0005 inches); the land diameter is smaller than that, usually about 0.0050 smaller, or a little more. That means that the first point of contact the bullet makes inside the barrel will be the lands. Learning where this point is can be a valuable thing.

Here’s a Hornady LNL OAL Gauge, along with the Hornady LNL Bullet Comparator. Midsouth has them. This appliance combination works along with a caliper and lets you determine the seating depth that touches the lands, and then gives a better way to measure and record it. Every serious handloader needs this setup! Get the angled version (as shown) because it’s more accurate than the straight one and easier to use.
Here’s a Hornady LNL OAL Gauge, along with the Hornady LNL Bullet Comparator. Midsouth has them. This appliance combination works along with a caliper and lets you determine the seating depth that touches the lands, and then gives a better way to measure and record it. Every serious handloader needs this setup! Get the angled version (as shown) because it’s more accurate than the straight one and easier to use.

We talked here about the essential forms a bullet ogive or nosecone can take, that some are more or less rounded or blunt, and also the gradual curve of a tangent-style ogive versus the more “spikey” secant style.

Usually (almost always) the nearer to the lands a bullet is the better the accuracy. “Jump” is the distance a bullet has to travel prior to engaging the lands. Different profiles will net different amounts of jump, and that led to my recommendation of a tangent-style bullet with no more than an 8-caliber ogive to give better results when it’s loaded to fit into a box magazine. Such a profile more easily enters the rifling and will have a longer bearing area (both mean less disturbance and a smoother trip through the bore).

There’s a valuable tool that will let you determine the relationship between your barrel and your bullet, which varies from barrel to barrel and bullet to bullet. Originally conceived by Tom Peterson of Stoney Point, it’s now offered through Hornady as the LNL O.A.L. Gauge.

This makes it easy to see how a comparator works, and also why it’s an asset: The tool measures at a point along the bullet ogive, avoiding the tip completely. Much more precise.
This makes it easy to see how a comparator works, and also why it’s an asset: The tool measures at a point along the bullet ogive, avoiding the tip completely. Much more precise.

The purpose of this tool is to show the overall cartridge length that has a bullet touching the lands. To get the most from this determination requires another gage, a bullet length comparator. What that does is measure the length of a bullet in a way that avoids the bullet tip coming into the equation. Especially in match-style hollowpoints, bullet tips are inconsistent. I’ve seen as much as 0.020 inches difference in some brands measuring base-to-tip in a box of 100. Closed-nose bullets, not so much, but they’re not perfect either.

There’s an opening in a comparator that comes to rest at a point along the bullet ogive, excluding the tip entirely. The diameter of the comparator opening may or may not coincide with land diameter, but it’s usually close. That doesn’t matter though. The combination of tools provides a pretty accurate picture. Of course, comparator-measured lengths won’t correspond in any way to tip-to-base lengths, but they provide a way to, as its name implies, compare different bullets. It’s the comparison, the net, that matters.

Use this tool set to determine the overall length that touches the lands, and then use the comparator afterward to determine the amount of difference when the bullets are seated to fit into a box magazine. Write it all down and we’ll talk more about making use of it next time…

 

Facebooktwittergoogle_plusredditpinterestlinkedintumblrmail

15 thoughts on “Bullet Seating Depth Determination”

  1. Another great article, thank you. I’ve been looking at these OAL gauges, comparing the old Stoney Point (Hornady) model to the Sinclair. Q: I thought the angled model of the Hornady which you recommend here was only for semi-autos? Hornady implies that if you want to check OAL on both a bolt and semi- auto that you need both the straight and angled gauges. (?)

    Also, I kind of like the look and quality of the stainless steel Sinclair gauge; are you familiar with that model? Seems a bit better built than the aluminum Hornady for about the same price, but I have no experience with either. You insert a bullet (through a cleaning rod guide) into the bore, move a stainless steel stop up to mark the spot (base of the bullet to the ogive) then chamber your brass and mark that spot with the second stop, then measure the outside distance with your calipers, then add the length of the bullet, (as I recall from the description.)

    I guess it’s the same idea as the Hornady, but I’m interested if you think one gives a more accurate measure than the other.

    I have the Sinclair hex nut comparator which doesn’t attach to your calipers, so have the second part of this covered, so just need the gauge. Q: do you think a comparator that attaches to the calipers is better, or are they about the same accuracy wise?

    Thanks for your thoughts on the two Hornady models vs. the Sinclair system.

    1. Forster products makes and excellent datum dial ammunition measuring system that also attaches to a caliper that produces great results !! Love mine .***Uncle Joe!!

  2. I’ve soft seated a bullet for the particular gun I’m building an accuracy load for then closed the bolt which pushes the bullet into the case and when you eject it (careful to not drop) measure the OAL and subtract .010″. Set seater to build every bullet the same. Don’t leave it on the land’s as you can increase pressure.

  3. Hmmmm. Interesting. I guess my method of measuring this is not as sophisticated as the two tools just mentioned, but I’ve had a lot of success with it.

    First, I take a cleaning rod and run it down the barrel until it makes contact with the bolt face. I then take a piece of electrician’s tape and wrap it around the rod right at the muzzle.

    Secondly, I take a specific bullet and drop it into the breech until it makes contact with the lands just forward of the firing chamber. I then take the same cleaning rod, and gently insert it down the barrel until it makes contact with the nose of the bullet. Note: it is important to make sure that the tip of the cleaning rod has a flat face. Once inserted, I take another piece of Electrician’s tape and wrap it around the rod right at the muzzle.

    Thirdly, I gently remove the cleaning rod, and measure the distance between the two forward edges of the pieces of tape with my dial caliper. This gives me the maximum Cartridge O.A.L. for that specific bullet.

    Admittedly, this is not as precise as using one of these tools, but it is close enough for my needs. I’m a Varmint Hunter, not a competition shooter, so I don’t have as much at stake as they do. I also admit that my method is kind of hokie, but it works for me.

    This being said, with my handloads, and when I’m doing my job as the shooter, my .223 averages slightly under 1/2″, the 6mm Remington averages 5/8″-3/4″, and the .17 Remington Fireball averages around 1/2″ when it’s not too windy.

  4. I have the straight Stoney Point OAL bullet comparator for many years and used it on various calibers. It works well for measuring base of case to where the bullet ogive contacts the rifle lands. It will also show you how shooting can change that due to barrel erosion. My one complaint is that consistency of measuring is extremely difficult to achieve. No matter how consistent one is in the process you can get varying results. The pressure applied to move the bullet to contact the lands is extremely difficult to do the same every time. Locking the set screw can change the length also. I also believe one should use a case fired in your rifle and sized with a minimum of shoulder setback then drilled and tapped on a lathe for more precision. Still with all of this results can vary. Just try it ten times and see the variations that you can get. Good luck!

  5. I reload for my 7mm Remington Mag.,used for hunting. After resizing my casing and training, what I do is take a permanent black marker and coat the entire bottom up to about 2/3 toward the tip. I then insert this bullet into the casing. If it is too loose, I dimple the neck ( just enough where it is inserted ) to prevent free movement. I then insert ( chamber) the round. Carefully eject this round and you will get the first contact distance for that particular bullet.
    I am sure your method of using the fancy technical instruments will be more exact, but I sometimes get lengths that, depending on type of bullet, can’t be box ( magazine ) fed. I have shot these single hand load only rounds and they are much more accurate than any manufacturer purchased cartridges i shoot. This includes Hornady,Remington, Winchester, and several others brands. Thank you for the great informative articles. Keep up the great work!

    1. You hit the nail with the point that you often can not load long enough to feed through the mags. In the long range community we are always chasing the sweet spot. Modified mag well and longer mags help. Also having a gun smith custom fit the length of your chamber is another big one. I shoot Berger 140 grain match hybrids in my Rem 260 and chased this problem for some time. .01 to .013 off of the lands seems to be a sweet spot for my rifle. But that put the OAL of the cartridge .025 longer than SAMMI specs so I bought Magpul bottom metal and AI mags. This allowed me to feed the longer more accurate ammo. For those of you just starting to reload the concepts presented here are definitely secrets worth knowing.

      1. Thanks for the comment. I read the Hornady and Nosler reloading manuals twice each, and refer back to them often. So, check TWICE, and fire once!!!
        I encourage many to hand load, as it is not only cheaper and fun, it gives us the most accurate cartridges possible. Just be safe. I like the 7mm as there are so many bullets and brands of powder, and casings. I like the 139 soft points for “self defensive long range rounds, and my other 7mm is great for the 150 grain ballistic tips for hunting deer. 800 yards is not out of the question with greater accuracy than factory cartridges. Good luck loading!

  6. Sorry, you lost me for a minute. You prep the case, then dye mark the bullet, and seat it in the case. If after ejection you find no imprint in the marked area, you re-seat the bullet a little longer?? You repeat until you capture a contact imprint and vola. Great tip, most of my purchases (.277 Hornady GMX ) are small lots (50) but it’s worth chasing, Thanks!

    1. You got it. I just insert the bullet about a quarter inch in and let the chamber do the rest. As the other poster said, do it several times, and check them all to get an average. I should have made that statement more clearly too.

  7. Since I started reloading in 2009 I have come across several articles as well as to my own way of thinking as to regarding this ” Land Loading”, for reducing jump. I have certainly found out that it improves the accuracy by a (guess) 300% . More or less like much like Robert, the way I had read and interrupted how to go about doing it was to set a case, THAT HAS BEEN FIRED IN THAT RIFLE ( most accurate head space) , cleaned and other wise left unchanged, perhaps with the slight exception for perhaps a light “crimp” if needed to hold the bullet in place, that is almost ready for primer, powder, and bullet…less the primer, powder, just insert the bullet.
    I’m probably going to say all the wrong words to describe this operation, hopefully you will understand and follow what I mean. I loosely insert the bullet in the cartridge case and making sure that it will not slide much and hold it’s position in the case, insert it in the chamber and bolt it into position as if ready to fire. I then eject as carefully as possible trying not to upset the relationship of the brass and the bullet that was seated to the lands of my rifle. I do repeat this operation several times on like length cases and bullets to ensure all measurements are VERY close together. An average is the best form of accuracy over several like items. At that time I transfer those measurements to paper, don’t trust the mind so much anymore, subtract another 0.002or 0.003 more to give me magazine clearance from OAL gathered at this point and that becomes my OAL for the “Land measurement” to reduce jump from 0.22 more or less to 0.001 jump to lands.

    1. Very well explained!… this is us poor folks way of doing what they did with all their fancy tools that we can’t afford! Thanks

    2. Yea, I should have said NO POWDER, NO PRIMER CASE.Thanks for bringing that most important factor up! Safety, Safety!!!

Comments are closed.