Determining Bullet Seating Depth | Top Rated Supplier of Firearm Reloading Equipment, Supplies, and Tools - Colt

Determining Bullet Seating Depth

Determining proper seating depth is probably one of the most important steps in preparing accurate, custom ammunition for your own rifle, unless you are loading for functionality in a magazine.

Finding the best bullet seating depth for a specific bullet to be used in an individual rifle is critical to the accuracy level that you are trying to attain. A huge benefit of handloading over factory loads is that you can change the overall length of your loaded rounds. The impact on accuracy can be very significant, probably more so than anything else you can do other than choosing the right bullet and powder combination. Let’s define a few terms first.

Terms
Case length – length of the case as measured from the case head to the case mouth. This length is needed for determining if the case needs to be trimmed. Case length is irrelevant for what we are currently discussing regarding bullet seating, so don’t get this confused with overall length (OAL).

Overall Length (OAL) – refers to the overall length of a completed cartridge. This length can be expressed in two different ways. First, this measurement can be expressed as the length of the cartridge when measured from the case head to the bullet tip. Second, it can be expressed as the overall length of the cartridge when measured from the case head to the ogive of the bullet. The first definition or value is useful if you are loading rounds to be fed from a magazine, such as in a hunting rifle or service rifle. Obviously, the relationship between the overall length of the completed rounds and the internal dimension of the magazine is critical for the rounds to feed smoothly through the magazine. The second definition is more important to those of us who want to obtain the best accuracy we can from our rifle and handloads. When we single-feed our rifles, we can extend the overall length of the cartridge beyond the internal dimension of a magazine, but we are still limited by the overall length of the throat and sometimes by the amount of seating surface on the bullet.

Bullet Jump – a common slang term for the distance between the ogive of the bullet and the point where that particular bullet will touch the rifling. As you hang around fellow accuracy nuts, you will constantly be asked, “how much “bullet jump” are you using?” You will also hear shooters talk about their bullets being .010” off of the rifling, same thing.

Bullet Ogive – bullet ogive is defined as the “curve of a bullet’s forward section” and can be expressed as a tangent ogive or secant ogive. A bullet with a tangent ogive is one that has the cylindrical surface of the bullet tangent to the curve of the point. A bullet with a secant ogive is one that has the cylindrical surface of the bullet secant to the curve of the head. Bullets with a secant ogive usually have a very obvious transition from the bearing surface to the ogive length. When we measure seating depth on loaded round rounds, we refer to where the major diameter of the bullet begins. This spot is approximately where the bullet will first contact the rifling and approximately where the bullet will contact a bullet comparator.

Throat/Leade – Interchangeable with leade. The throat is located just forward of the chamber and is the tapered entrance leading to where the rifling begins in the barrel. The throat is an area of the rifling that has been relieved/machined to allow the bullet clearance prior to reaching the rifling. This is sometimes referred to as the unrifled section of the bore or free bore.

Rifling – Rifling refers to the parallel spiral groves cut into the barrel of the firearm. Rifling imparts the spin on the bullet.

Lands – This is the raised portion of the barrel that remains after the grooves have been cut or pressed into the barrel to make the rifling. Typically, the lands diameter of a .30 caliber barrel will be approximately 0.298” to 0.300”.

Grooves – The spiral cuts in the barrel that are made to create the lands, which together induce the bullet spin. Typically, the groove diameter in a .30 caliber barrel will measure approximately 0.307” to 0.3075”.

Everything we discuss pertaining to bullet seating from here forward is going to be about the actual overall length of our finished loaded round with respect to the bullet touching the beginning of the rifling. The measurement of this overall length with the bullet touching the rifling will be our initial reference point.

One point that many reloaders have difficulty understanding about bullet seating depth and OAL is that the overall length of a round assembled with the bullet just touching the rifling is going to be a different length than the same round with a different bullet that is also touching the rifling. Each bullet has a different shape and with the throat leading to the rifling being cut on a taper, the point of contact on the bullet is different. This is true to a lesser extent on two of the same style bullets. By using a bullet comparator, you can measure this difference within two manufacturing lots of the same bullet and often even on bullets within the same box.

The ammunition that you assemble will have a certain overall length that performs best for your particular rifle. Finding the best overall length is done during load development, but to begin with, we want to determine the length of a round with the bullet just touching the rifling. There is more than one method to find this length, but only a couple of ways that we recommend. There are only a few tools made to determine this measurement.

One method of finding the overall length is to use a Sinclair Seating Depth Tool. This tool can be used in most bolt-action rifles, single-shot rifles like the Ruger #1 and #3, Thompson Contenders/Encores, and some semi-autos like the AR-15/M16. The Sinclair tool is inserted through the chamber end of the action and obtains the desired overall length measurement in a two-step operation. It is an extremely accurate method when performed using a fired unsized cartridge case that was shot in the same barrel you are working with. The tool consists of a stainless steel measuring rod, two stop collars with parallel surfaces, and an action guide. Below are the steps for obtaining the measurement in a bolt-action rifle, but the steps for the other types of rifles are essentially the same.

Using a Sinclair Seating Depth Tool with Comparator

    1. Remove the bolt from the rifle and set it aside.
    2. Drop the bullet you want to use into the chamber making sure that the point drops straight in and that the bullet is resting in the throat. This step is best done with the unloaded rifle standing on end with the muzzle on a piece of carpet or a towel.
    3. Place the correct size guide into the action and lock it in place like a bolt.
    4. Insert the measuring rod into the guide and move it forward until the end contacts the base of the bullet. Push lightly to ensure the bullet is fully contacting the rifling.
    5. Slide one of the stop collars with the large diameter forward onto the measuring rod until it contacts the back of the bolt guide.
    6. While maintaining light pressure on the bullet, make sure the guide handle is fully forward and then lock the stop collar in place with the thumbscrew. Remove the entire tool from the rifle.
    7. Next, remove the bullet from the rifle (SAVE THIS BULLET). It will probably be stuck lightly into the rifling so you can push it out from the muzzle end with a cleaning rod or, better yet, drop a smaller caliber bullet down the barrel and it will knock it loose. You can stick a piece of tissue or paper towel into the rear of the action to catch the bullet. The dropped bullet method works well unless you are measuring a .22 caliber. Most of us usually do not have .17 caliber or .20 caliber bullets at hand.
    8. Insert a case into the chamber by hand (preferably use a fired, unsized case - leave the spent primer in as long as it is flush or slightly below flush). Push the case in most of the way with your finger.
    9. Place the guide back into the action. Maintain the same orientation of the measuring rod as it was in step 4 and slide the second collar onto the leading end of the rod prior to inserting it into the guide.
    10. Make sure the large diameter side of the stop collar goes on first so the smaller diameter is oriented towards the end of the measuring rod that will be inserted into the guide. The two stop collars should be positioned so the large diameters are facing each other. If not, we messed up somewhere!
    11. Slide the assembly into the guide until the end of the rod comes in contact with the case head. Use the measuring rod to push the case firmly into the chamber until the shoulder seats tightly.
    12. Push the second stop collar up against the guide while maintaining pressure against the case and lock the thumbscrew.
 
  1. 13. Remove the measuring tool from the action along with the guide. You can push the case out with a cleaning rod or simply insert your bolt, the extractor should engage the extractor groove, and you can remove the case.
  2. Using calipers, measure the distance between the stop collars by placing the blades on the outside edge of the large diameter. Make sure you keep the caliper blades in full contact with the stop collars to get a good measurement. We may remeasure a couple of times to make sure we are getting a consistent measurement. Write this measurement down. It is the distance from the bolt face to the base of the particular bullet you used.
  3. To get the total overall length, take the measurement from step 14 and add it to the length of the bullet. Measure the bullet from base to ogive using a bullet comparator. Save this bullet in a tube or small plastic bag.

Now you have an overall length of a round that touches the rifling for that particular bullet and that barrel. Let’s look at a tool originally made by Stoney Point Products (now Hornady) that does about the same job in a different way.

The Hornady/Stoney Point OAL Gauge is an easy tool to use and, we use it occasionally. It is not as accurate as the Sinclair tool because it uses factory new cases as part of the construction of the tool. The variations in headspace between the factory sized cases and your rifle’s chamber are what can cause the inaccuracy. But, it is a good safe tool to use and we do recommend it to a lot of handloaders because it is very simple to use. The Hornady tool requires you to purchase specially modified cases for each cartridge you reload, but they are relatively inexpensive.

The Hornady/Stoney Point tool uses a special modified case that threads onto a hollow tube containing a lockable pushrod that runs through the center. Hornady makes the modified cases for most common cartridges (approximately 87 at the time of this writing). The modified cases are factory new cases that have been drilled and tapped through the case head so they can mount to the tool. They also have the case neck opened up a few thousandths over bullet diameter. If you load wildcat cartridges you can send a couple of fired cartridges to Hornady and they will drill and tap them for you (for a small fee).

Hornady OAL Lock N' Load for Bolt Guns and Semi-Autos
Hornady OAL Lock N' Load for bolt guns and semi-autos

Using a Hornady OAL Gauge with Sinclair Comparator

  1. Begin by threading the modified case onto the OAL Gauge, and then loosen the thumbscrew on the side of the tube. This thumbscrew holds the pushrod in place. Withdraw the pushrod into the tube and insert a bullet into the cartridge case neck. The pushrod should be positioned so a small portion of the bullet is sticking out of the case neck. This is not critical, just don’t have it too far out of the case neck because will fall out when you try to insert the tool into your action.
  2. Remove your rifle bolt and insert the complete tool into the action, case and bullet first.
  3. Push the back end of the tube firmly so the modified case seats into the chamber.
  4. While holding the tube in place, gently slide the pushrod forward until you feel the bullet meet the rifling. Gentle pressure on the pushrod is enough. Too much pressure and you will actually begin seating the bullet partway into the rifling, which will result in an erroneous measurement.
  5. While maintaining pressure on both the tube and the pushrod, turn the thumbscrew to lock the pushrod in place.
  6. Withdraw the entire assembly from the firearm. The bullet will probably stick in the rifling. Push the bullet out with a cleaning rod or drop a smaller caliber bullet down the barrel to dislodge it as described earlier.
  7. Place the bullet back into the neck of the modified case and measure the overall length of the cartridge with a bullet comparator. This measurement is the length of a loaded round with the bullet touching the rifling (for that particular bullet). Write this measurement down and save the bullet as a reference.

There are other ways to measure seating depth from the bolt face to the bullet touching the rifling. Both methods are less accurate than the two above and can be dangerous. The first is the older method of smoking the surface of the bullet with a candle and then trying different seating depths until you can just begin to see the engraving of the rifling on the smoked bullet surface. This can be time consuming and inaccurate. A smoked bullet will usually not show the rifling marks until it is forced into the rifling at least 0.005” to 0.010”. If the rifling engraves the bullet to the point where the marks appear horizontal to the bullet’s center line, you can be sure that the bullet has been forced into the rifling at least 0.015” to 0.020”. If for some reason, you want to use this method, MAKE SURE YOU USE A DUMMY ROUND WITH NO POWDER AND NO PRIMER. NEVER, EVER CHAMBER A LOADED ROUND WHEN MEASURING SEATING DEPTH. There have been several accidents involving handloaders using this method. Never place a loaded round into your firearm unless your weapon is pointed downrange on a live fire range.

Another method of obtaining the overall length is to place a bullet into the chamber (bullet only) and hold it against the throat with a piece of dowel rod or a pencil. Then insert a cleaning rod or a long wooden dowel rod into the muzzle until it contacts the tip of the bullet. Draw a line on the rod at the muzzle. Push the bullet out and replace the bolt so it is in the closed position (make sure it is empty). Leave the bolt in the cocked position so the firing pin doesn’t affect the measurement. Push the dowel or cleaning rod down the barrel until it contacts the bolt face. A second line is drawn on the rod at the muzzle. The difference between the two drawn lines is the overall length of the round with that particular bullet touching the rifling. This method is not very accurate due to the difficulty of getting your pencil lines exactly at the muzzle and the inaccuracy of measuring between pencil lines. A pencil line itself can measure around 0.030” to 0.050”. This same method can be done using small stop collars placed on the rod and measuring the distance between them. There are several documented accidents with shooters leaving a rod in the barrel after using this method. One of these accidents resulted in severe backpressure, which unfortunately resulted in the shooter’s death.

We highly recommend using the Sinclair or Hornady tool. These are the most accurate and safest tools to use. Regardless of the method used, you now have an overall length of the cartridge with the bullet touching the rifling. You can go ahead and set up the seater so it is ready to seat bullets after you throw powder. Set up the bullet seater using the measurements obtained and the bullets used to acquire the measurements.

First, make up a dummy round with the bullet used in the measurements. A dummy round is a cartridge with no primer and no powder. The cases you reject during sorting are excellent for this task. The dummy round you build will serve as your benchmark for future adjustments to seating depth. You can also use this dummy round to measure throat erosion (see highlighted area at the end of this article). Make the dummy round the same length as the measured overall length. Let’s setup the seater to make the dummy round.

Regardless of the bullet seater, run the bullet seating stem up high enough so that at the top of the stroke on your press the bullet is just barely seated into the case. If you are using a hand die run the stem up high enough so that after fully compressing the seating stem cap the bullet is barely seated. Begin adjusting the seater stem down, each time checking the overall length of the round with the same bullet comparator you used earlier. The goal is to get the seater adjusted so the dummy round measures the same as the measurement you obtained using your seating depth tool. This task is relatively easy if you have a seater that has a micrometer head built into it. If you don’t, use the following table, which shows some of the more commonly used threads per inch (TPI) used by many die makers when machining the threads on the seating stem; the TPI can be converted to the vertical movement of the seating stem when it is rotated. For example, a 90-degree (1/4 turn) clockwise turn on a standard Redding seating stem (20 TPI) is equivalent to about 0.013” of downward bullet movement.

Threads Per Inch Chart/Degrees of Rotation

 

TPI
90°
180°
270°
360°
16
.016"
.031"
.047"
.063"
18
.014"
.028"
.042"
.056"
20
.013"
.025"
.038"
.050"
24
.010"
.021"
.031"
.042"
28
.009"
.018"
.027"
.036"
32
.008"
.016"
.023"
.031"

 

An easy way to adjust seating depth on hand bullet seaters is referenced below in the shaded box.

Adjusting Seating Depth on Hand Style Bullet Seaters
If you are using hand dies to seat bullets and your die doesn’t have a micrometer head, there is a relatively quick and accurate way to adjust your seating die. Pull the cap and stem out of the seating die, measure the length of the cap and stem with your calipers and record the measurement. Loosen the setscrew that holds the stem in place, rotate the stem in the direction needed to either increase or shorten the overall length, then measure the cap and stem again. If you are trying to increase the overall length, the cap and stem should be shorter than the original noted length by the amount of change needed. If you are trying to shorten the overall length, the cap and stem should measure longer than the original length by the amount of change needed.

Once you have the seater set to the overall length so the bullet is just touching the rifling and your dummy round is constructed, you can move on. Write the measurement down with a fine-tip felt marker on the dummy round. If it measures 2.065”, write that on the side of the case. Also, write the date and the words “On rifling” or “OAL”. This will remind you later what the dummy round is for. Put that round away in a safe place. An empty pill bottle or cleaning brush tube is an excellent storage container. You may want to reference this baseline round later for checking throat erosion or further adjustments to seating depth. Write this information down in your logbook. Record the date, the bullet (brand, weight, type, and lot#), and the number of rounds through the barrel at the time of measurement. If you have more than one bullet comparator make sure your records indicate the comparator you used because a year from now you won’t remember - it will make a difference.

Initial Bullet Seating Depth
You have determined what the overall length of a round is when the bullet is touching the rifling. That is your baseline for that particular bullet. You don’t necessarily want to begin with that particular seating depth. In a few cartridges (especially wildcats), fire-forming is necessary, and this is usually best done with the bullet just touching the rifling. In most cartridges the forming that occurs in the chamber is minor and no special seating depth is needed to help “blow out” the cartridge.

We suggest starting out at 0.010” to 0.015” off of the rifling for .22 caliber centerfire cartridges and at about 0.010” off for larger calibers. This is just an initial starting point; each rifle and cartridge will be different. For your first loads in load testing, we suggest assembling all of your rounds with the same seating depth and vary your powder selection and/or bullet selection. Be careful about starting with the bullet touching the rifling, especially with initial loads for the cartridge. If the bullet is touching the rifling, chamber pressures will definitely increase.

 

CAUTION
Seating a bullet to touch the rifling will normally increase the chamber pressure with respect to the loads you are currently using. Reduce your normal load and work your way up or down until you develop a load that is both accurate and safe. Be careful to not exceed the measured overall length.

 

Set up your seater now so it is ready to use after throwing powder. Let’s use the example of setting the seating depth so the bullet is 0.015” off the rifling. Use the measurement you obtained for touching the rifling and adjust the bullet seater so it will produce loaded rounds 0.015” shorter than the OAL measurement. Use the thread pitch chart above or your seater’s micrometer if it has one. Use your bullet comparator in this step instead of measuring to the bullet tip. Either seat a bullet into an empty case and measure or check the measurement on the first live load you make and adjust accordingly. You can always earmark these first couple of rounds as “foulers” if the bullet seating depth comes out a little short.

Now you are ready to go and do some testing at the range. If you have the time and enjoy your time at the range; a Sinclair Arbor Press and Wilson Bullet Seater, a powder measure with stand, scale, and a bullet comparator with calipers makes it easy to try different loads at the range. You can prepare a batch of cases and have them ready to try different powders, bullets, and of course seating depth changes. This is an outstanding way to spend a quiet morning or afternoon at the range!

 

Using Seating Depth to Measure Throat Erosion
You can easily measure and observe throat erosion using the Sinclair tool or the Hornady tool for finding overall length. Regardless of which tool you use, record the overall length by building a dummy round or record the length and save the bullet used to find the overall length. After you have fired a number of rounds through your barrel, use the same bullet to measure the overall length again. The overall length should be longer due to the throat erosion that has occurred. The barrel steel, the cartridge, the powder, and how hot your load is will all affect the amount of throat erosion which occurs. In most cases, changing your overall length to maintain the same relationship to the throat will keep your rifle shooting accurately.