Bullet seating, basic and advanced
Before you shove a bullet into the neck of a rifle case, you need to at least have a basic understanding of how things work. At face value, it seems like a very elementary action but if you dig a little deeper you’ll see that there’s a little more to it than meets the eye. For plinking ammo, how deep you seat the bullet is ultimately determined by magazine limitations. For example, ammo for an AR15 must be 2.260” or less, measured from the tip of the bullet to the base of the case. For rifles whose magazine will accept ammo of virtually any length, then the overall length is either determined by the distance from the bolt face to the beginning of the rifling or something shorter set by the hand loader. This will make much more sense as you read further. We’re going to talk about some different strategies for determining final seating depth and I’ll explain how accuracy and consistency is affected by variations in seating depth.
Let’s start with some basics. Don’t over-think seating the bullets. At least not for basic, plinking ammo. Only a couple of things need to be considered. Magazine limitations and/or the published overall length. Bullet seating depth plays a vital role in initial pressures when the round is fired. Naturally, the deeper the bullet is seated, the less internal volume there is available inside the case. The smaller that space, the higher the initial pressure will be. In nearly every situation, when a beginner asks me about a starting seating depth, I tell them to use the length their manual gives them. Every manual will list a “C.O.L.” or Complete Overall Length (or Cartridge Overall Length). In most cases, that’s the seating depth they used to gather the data that’s published. In nearly every situation, acceptable accuracy can be achieved at that length.
There are 2 ways of measuring completed rounds. One is Tip-To-Base and the other is ‘Base-To-Ogive’. We’ll dig into the ogive of a bullet and how to measure to that part of the bullet, as well as the benefits of measuring there, later in this article. For now, we’ll talk about measuring at the tip. It’s much less confusing to the beginner. It’s done exactly as the words describe, from the base of the case to the tip of the bullet. There is a negative side of measuring this way, however. If you were to measure each projectile individually with your caliper, you’ll see that there will be a variation in bullet lengths. The blame for this lies in the tips, themselves. They may vary as much as .010” or more. The dimensions of just the tip of a bullet is fairly unimportant. It plays very little role in the performance of the bullet in flight other than providing aerodynamics. Just like the measurements varied when you measured the projectiles themselves, when you seat bullets in the cases and measure Tip-To-Base, your measurements will also vary. This is normal and no need for concern.
The lengths given in the data manuals will always be ‘Tip-To-Base’ and for the rest of this article, I’ll refer to that as the TTB measurement. Referring back to the AR15, the 2.260” TTB is the maximum length the magazine will accept. It would be wise, when seating .223 bullets for that magazine, to have a TTB measurement shorter than that to allow for that variation in tip lengths. Personally, I start with 2.245”. That allows me .015”, therefore the longer bullets should have enough room to fit in the magazine without risk of being too long. The magazine limitations apply to any rifle, though, not just AR15s. For the most part, the lengths given in the manuals take magazine limitations into consideration. To sum that up, if you will use the book length, you’ll rarely encounter any problems related to overall length.
Basically, when you’ve determined the length to begin with, you’re ready to seat your bullets. At this point, your case prep should be 100% complete, primers in place, powder charges dropped, burs removed from the necks, etc. Assuming so, simply set the seating die up according to the instructions that came with your die set. I recommend that once the die is screwed down and locked into place, you back the seating adjustment up a few turns. This will seat the bullets longer than you need, but it’s easier to continue adjusting to seat them deeper until you reach your goal length than to seat them too deep and have to pull them back out some. So place a primed and charged case in the shell holder and place a bullet in the mouth of the case. You may have to hold it in place until the bullet is well inside the case in some cases. Just be careful and go slow so you don’t get your finger damaged in the process. Then you just raise the ram all the way up and then lower it. You should have felt the resistance when the bullet reached the seater plug in the die. Measure from TTB and compare that to your goal length. Now you simply turn the adjustment knob down in small increments and run the cartridge back into the die repeatedly until you reach the desired overall length. That round is now complete. If you used the book length, you should have no problems with this round fitting in any magazine and safely chambering in any rifle chambered for that cartridge. The bullet should not reach the rifling at that length. This is a very basic bullet seating technique. One you’ve gotten that first round to the correct length, seat the bullet in the next case and measure it. It might or might not be identical to the previous round. This is where you’ll see some variations in lengths. I recommend measuring the first few rounds and as long as they’re within .010” or so, proceed with the rest of that batch and measure every 10th round just to make sure your equipment is still set and locked. It’s that simple.
Now we’ll get into some more advanced techniques in seating depths as well as strategies to use when determining a final seating depth. I’m going to try to make this easy to understand and make sense. We’re through measuring at the tip of the bullet now. From here on out, we’re going to measure from the ‘Base-To-Ogive’, or BTO. Instead of using the length listed in the manuals, now we’re going to determine our own length. We’ll take a measurement inside the rifle chamber and determine how long the completed round can be before the bullet engages the rifling. We’ll document that measurement and seat bullets at lengths based on that measurement. Ultimately, some rifles will still limit the overall lengths of the completed ammo due to the length of the magazine, so once you determine your maximum overall length, you’ll still need to make sure the magazine will accept ammo that length.
To determine the length of the cartridge at which it touches the rifling, there are 2 methods. There may be other ways but if so, they’re going to be similar to these methods I’m going to talk about. Th first method is to use an “overall length gauge”. Hornady makes one that works great. It’s basically a long tube with male threads on one end. Inside the tube is a rod that freely slides back and forth. Then there’s a locking knob that will secure the rod in place. In addition to the gauge, you’ll need to buy the modified case for the caliber you’re working on. It’ll basically be a rifle case with the base drilled and tapped so that it threads onto the tube. You thread the modified case onto the tube and place a projectile in the mouth of the case. The rod inside will slide forward and contact that bullet so back that rod out so that the bullet can be inserted into the case neck. To see how it works, move the rod in and out to see how it forces the bullet out longer and how it allows it to be pushed deeper into the case. Once you see how it works, remove the bolt from your rifle (if applicable) and insert the gauge with the modified case and bullet attached into the chamber. Tremendous force is absolutely not necessary. Just push the modified case forward into the chamber until it stops. Hold th gauge in place and then slowly and carefully push the rod forward. This will push the bullet forward and when you feel it stop, you stop pushing. You’re pushing the bullet out longer until it reaches the rifling. You don’t want it to be forced into the rifling, but rather just barely touching the lands. When you feel the bullet engage the rifling, tighten the locking knob to hold the rod in place and remove the assembly from the rifle. It’s possibly for the bullet to occasionally become slightly stuck in the rifling when you remove the gauge and if this happens, simply push the bullet out with a wooden dowel or cleaning rod. With the assembly out of the rifle, place the bullet in the case mouth again. It will be stopped by the rod inside the tool. Then simply measure that length with your caliper. The gauge has a window at the base of the modified case that allows one jaw of the caliper to measure on the base of the case. Use your bullet comparator to measure on the ogive of the bullet. Write that measurement down. Now I recommend repeating this process a few times just to be sure you get the same results. Once you’re comfortable that you’re getting an accurate measurement, then you’re through and you’ve now determined your maximum cartridge length. (Note: this process will need to be repeated for every different kind of bullet. If you get this measurement with a 53gr V-MAX, then if you switch to a different bullet, such as the 52gr A-MAXs, this will need to be redone. No two bullet designs will produce the same measurement if they do, it’s merely a coincidence.)
The 2nd method is to make your own modified case for the caliber you’re loading. To do this, sacrifice an empty case and resize the case. FL resizing is preferred so that the case isn’t too tight in the chamber. Then use a small machinists file, or equivalent, and from the inside of the neck, file a slit in the neck. The slit needs to extend from the end of the case neck to the beginning of the shoulder. The purpose of this slit is to partially relieve the neck tension the case will have on a bullet. To use this tool, simply insert one of the bullets you’re going to be using into the case neck. Leave it considerably long by only pushing it in far enough to make it stay in place. Place the case and bullet into the chamber and very slowly and carefully close the bolt and lock it. Then, just as slowly and carefully open the bolt and remove the modified case and bullet. Now measure the length. The rifling will push the bullet into the case as you close the bolt, giving you your maximum overall length to the rifling. Again, repeat this until you’re convinced that you’re getting an accurate reading. Once you determine the max length, you’re done and can proceed to the next step.
Now that we know how long a round can be before it reaches the rifling, we can come up with a length to begin testing. There’s no rule etched in stone that tells you where to start, but before you start, let me warn you that loading cartridges so that the bullet is “jammed” into the rifling when a round is chambered can cause tremendous pressure spikes so I recommend backing off that length by as much as .050” and begin testing for your best powder charge. (This testing will be covered in another article). Once you determine your most stable powder charge, then you can start adjusting the seating depth to tune the load.
Before you totally settle on a length, one more thing you need to know is how much of the bullet is in the case neck at that length. As a ‘Rule of Thumb’, you want at least the diameter of the bullet in the neck. Example: if you’re loading for a 6.5 Creedmore, you’ll want at least .264” of the bullet gripped by the neck. As with a lot of things in the reloading world, this is not a definite rule. It’s simply a good guide. In tuning the load, the overall length will be adjusted which means the amount of the bullet in the case will change. That’s ok as long as there’s enough bullet in the case to keep it firmly held in place.
Now we’re going to start wading into the fine tuning of a load by adjusting the seating depth and in the process, I’ll explain why length adjustments change the performance of the load and affect the group sizes. When a round is fired, a lot happens within the rifle case as well as throughout the entire rifle. At the very root of everything is something called pressure. The powder charge is determined by pressure. The seating depth is determined indirectly by pressure. Safety precautions are all brought to us because of pressure. Pressure is what makes the bullet travel from its motionless position to its final destination. The primer ignites the powder through the flash hole and, since the powder is contained in small confinements, the powder burns very quickly, generating tremendous pressure. This pressure begins inside the case but it’s contained by the thick steel around the chamber as well as the walls of the entire barrel. As the powder ignites and burns, pressure is built in literally a few short milliseconds. It happens so fast that we often refer to it as an explosion when, in fact it’s more like a controlled burn. Regardless of what we call it, it’s fast and it’s violent. This sudden pressure creates vibrations and harmonics within the entire rifle. Around the same time, the bullet begins to move in the neck. It takes a certain amount of pressure to get the bullet moving and as it moves, the internal space, or volume, increases. Now, the powder continues to burn so to say the pressure stops increasing as the bullet moves would probably be incorrect, but it does affect the rate at which it increases. The real pressure spike occurs when the bullet makes it to the rifling. The bullet fits very tightly in the bore and if you were to try to force a bullet into, and through, the bore, you’d definitely have your work cut out for you. I’ll save you some trouble and say “Don’t even try”. But if you did, you’d see how tough the task is for the powder charge. Just stay with me. This will start to make perfect sense very soon. So the powder is generating tens of thousands of pounds of pressure per square inch. Where the bullet starts this journey in relation to the rifling greatly affects the behavior of the rifle and ammo together.
I’ll refer you back to the beginning of this article and mention the ammo loaded at the length in the data manuals. In many rifles, that length will provide quite a long jump to the rifling. There are exceptions to this, of course, but for the most part, that’s a true statement. With the long jump, the bullet gets a running start at the rifling. This affects pressure differently than of the bullet were already touching the rifling. Picture this: you have a glass table. If you hold a brick 18 inches above that table and dropped it, chances are, the table will break. Let’s try a different scenario. On that glass table, gently place the brick on the table. Did it break the glass? Probably not. In order for that brick to break the glass now, you’ll probably have to stand on the brick and maybe even jump to get it to break. In these two scenarios, which brick required the most pressure to make it go through the glass table? It was the one that you sat on the table, right? The same theory applies to bullets jumping to the rifling as opposed to starting against the rifling. Make sense now?
So which way is better? Well, that subject is always up for debate. I have a Remington 700sps chambered in .204 Ruger that has a tremendously long throat. If I tried to seat the bullets so that they touched the rifling, the bullet wouldn’t even reach the rifle case, yet that rifle will shoot .00”-.04” groups regularly at 100 yards. I can’t argue with that kind of accuracy. I also have a Savage Model 11 chambered in .260Rem that has a much shorter throat. After all my fine tuning, I load that ammo so that the bullets jam the rifling by .010”. That rifle shoots 1/8MOA, and sometimes even better than that.
Some bullets are really picky about their seating depth while others aren’t picky at all. It depends on the ogive design. I mentioned the ogive earlier in this article but I’ll go into a little more detail about the different designs. The three designs are the Secant Ogive, Tangent Ogive and the Hybrid Ogive. The Tangent Ogive design makes the bullet easier to tune because it’s less sensitive to seating depth/jump to the rifling. The reason is because the transition from the tapered section to the bearing surface is much more subtle, helping it center itself in the bore gradually as it engages the rifling. Most hunting bullets have this ogive design. The Secant Ogive has less curve between the Meplat (tip) of the bullet to the bearing surface, making it an abrupt transition from one section to the other. This design has less ‘help’ centering in the bore as it contacts the rifling. Most match grade, target bullets have this ogive because it creates less aerodynamic drag as it flies, greatly increasing the ballistic coefficient of the bullet. The Hybrid Ogive is pretty self-explanatory. It employs a combination of the Tangent and Secant Ogive. This kind of provides the best of both worlds. It allows manufacturers to produce a bullet designed for hunting situations as well as increasing the BC of the bullet, resulting in improved accuracy at longer distances and less sensitivity to seating depths.
I’ll give a hint, now, to why you’d want to have a bullet jam the rifling when chambered. This might step away from bullet seating briefly but I have to include other things in this scenario to make it make sense. We already know the three methods of sizing brass, right? FL sizing, neck sizing only and partial FL sizing (bumping the shoulders). One extremely important factor in ultra-accurate ammo is for each round to be perfectly centered in the chamber and bore when chambered. Imagine a centerline through the rifle barrel, starting at the rear of the bolt and firing pin. You want the chambered cartridge to be perfectly lined up with that imaginary line. This is one reason why neck sizing only increases accuracy. The case fits tightly in the chamber, therefore it’s centered in the chamber when the bolt is closed. If you choose to FL size or bump the shoulders, then you conform the brass more to the centerline of your die rather than the rifle. Yes, dies are made to high standards and produce excellent ammo but when you’re wanting precision accuracy, you want the ammo 100% in tune with your rifle, not your dies. That’s why I neck size only. With FL sizing, you’re now depending on the bolt face and extractor to hold the loaded round in line with that centerline of the rifle. There’s nothing on the front end of the round to support it and hold it in place. Granted, the clearance is very, very small, but the fact is, it’ll tend to ‘sag’ slightly without any front support. So in that scenario, if the bullet is seated to a length that puts it into the rifling when the chamber is locked, then it will lift and support the front of the round and center itself in the bore. Without being centered with the bore, when the round is fired, the bullet will slam into the rifling ‘off-center’. This means the bullet will not hit the rifling on all 360° at the same time. By hitting the rifling off-center, the bullet now “wobbles” into the bore, for lack of a better term. This happens violently, too, causing tremendous unwanted vibrations throughout the barrel as the bullet travels toward the muzzle, not to mention how this will deform the jacket and the bullet, thus affecting the external ballistics. So on the bottom line, loading the ammo so that the bullets engage the rifling greatly reduces vibrations in the rifle when it’s fired. This greatly increases consistency and accuracy.
Now we’ll talk about seating bullets so that they don’t reach the rifling. By doing so, you leave yourself the option to fine tune with depth adjustments. So you have your initial load testing done and you’ve found your optimal charge weight, but the ammo still doesn’t group as tightly as you’d like. Now what? As long as you have room in your magazine for length adjustments, you can start tweaking the seating depth. Let’s assume the bullet is .050” from the rifling when the round is chambered. That number is strictly for example. The same steps will apply regardless of your actual distance to the rifling. When making seating depth adjustments, a rule that I go by is the faster the bullet speed, the smaller the increments you adjust the seating depth. For example, in my 204s, I adjust the seating depth by just .003” to get noticeable changes in the group sizes. In my 270s, I make .015” adjustments, and so on. There’s no absolute rule that tells you exactly how much to adjust it. You can basically adjust it as much as it takes to get the desired results. In the small, fast calibers, that ‘sweet spot’ will be much more narrow than the larger, slower calibers.
The reason seating depth changes makes the ammo shoot differently is because it changes the pressure, among a few other things. As you extend the bullet longer, you increase the volume in the case and, of course, if you seat it deeper, you decrease that volume. Both will change that initial pressure when the round is fired. This change in pressure changes the vibrations and harmonics of the rifle. When the round is fired, a shock wave quickly and somewhat violently travels to the muzzle of the rifle and returns to the chamber. This shock wave causes the barrel to ‘whip’. This whipping action, naturally, makes the muzzle of the barrel move from its original position. If the muzzle is off-center when the bullet leaves the barrel, it will be sent in the direction the muzzle is pointed. The whipping action is not up and down, nor left and right. It will whip in any direction based on how the shock wave affects it. The ultimate goal of the reloader is to time the bullets’ exit so that they leave when the muzzle is back at its original position. Changing the seating depth will not only change that initial pressure, thus changing that shock wave, but it also changes the timing of the bullets’ exit. You can’t tell by watching the barrel if you need the bullet to leave sooner or later, so you simply make adjustments longer and shorter and let the rifle tell you which way works best.
Part of what makes hand-loaded ammo perform better than factory loaded ammo is because we, as reloaders, have the liberty to do practically anything we want and need to get different, and hopefully better results. We change powders or the amount of powder, the primers, the brass manufacturer, the type or manufacturer of bullets and even how deep or shallow we seat the bullets. Regardless of what ingredients we use, the pressures generated when a given round is fired greatly affect the consistency at which they leave the muzzle. Adjusting the seating depth is only a small step in a longer journey on the road to precision accuracy!
Jason Ray, Nacogdoches, Texas