Cartridge Overall Length (Seating Depth)
The overall length of the cartridge is a very important detail. That’s why handloading manuals list the overall length (seating depth) for every different bullet. Overall length affects gun function and safety. If the bullet is seated too long, the cartridge might get stuck in the magazine. Or the cartridge might jam during feeding because the bullet engages the riflings before the case is far enough in the chamber. This will prevent the slide from going all the way forward and the gun won't fire. If the cartridge is too short it can cause feeding failures if the point of the cartridge nose-dives and hits the feed ramp lower than usual and at a sharper angle.
Seating depth also affects pressure. The deeper the bullet, the higher the pressure. Normally this is balanced with a gunpowder charge appropriate for the bullet weight and seating depth. But when the cartridge is loaded to maximum pressure, and the bullet is seated significantly deeper than it should be, a dramatic increase in pressure can skyrocket to dangerous levels and this risks the chance of a case blowout, or worse.
When loading any cartridge, the golden rule is: not too long, not too short.
Generally speaking, the length of the loaded round should be as long as possible to enhance feeding reliability, but not so long that it jams in the magazine or in the chamber. And the bullet should not be seated so deep that it hampers feeding reliability or pushes pressure too high.
In order to spare the casual reader from wading through all my data to get the simple answer, I'm going to begin this section with recommended cartridge overall lengths with common bullet shapes. These are shown in Table 1. The brief explanation is that flat nose bullets (like hollow point bullets) generally have to be seated deeper than round nose bullets in order to fit in the magazine. The Sierra jacketed hollow point bullets must be seated especially deep because of their unique design in order to fit in the typical 38 Super chamber. If you want a better understanding of why seating depth is different for different bullets shapes, read the rest of this page.
Table 1: Suggested Overall Length (Seating Depth) with Round Nose and Flat Nose Bullets in the 38 Super
* 1.280 inches is the maximum overall length for the 38 Super cartridge. JHP = Jacketed Hollow Point. ** You have to test the proper length for your gun's chamber. To do this, remove the barrel from your gun. Load a single cartridge rather long, say 1.250. Drop it in the chamber to see if it goes all the way in and if it engages the riflings (it gets stuck). If it does, seat the bullet deeper and continue this test until the cartridge goes all the way in the chamber and does not get stuck. Then measure and record this length. Different barrels might have different tolerances for the Sierra and other bullets.
Seating Depth and Bullet Shape
The Sporting Arms and Ammunition Manufacturers' Institute, Inc. (SAAMI) established maximum overall length for the 38 Super is set at 1.280 inches. It’s not unreasonable to expect gun manufacturers to design their magazines around this established standard. Ammunition manufacturers also design their product around this standard and don't load ammo longer than the maximum length. The overall length of factory 38 Super ammunition can be found here on this website. The longest overall length of the factory ammunition that I measured was the Frangible Bullets load at 1.279 inches. Other loadings were not far behind in length.
Round nose bullets can be seated to the maximun length and will (or at least should) fit and feed in all magazines. However, flat nose bullets like hollow point bullets generally must be seated deeper in order to fit in the magazine. Why do different bullets types require different seating depths?
#1. The front surface of the magazine is round.
#2. Cartridges sit in the magazine at an angle.
Lets look at each of these in some detail.
#1. The front surface of the magazine is round. This is obvious for single column magazines. Many double column magazines have a flat front surface, but the edges are round. Round nose bullets fit well in the round contour and don’t need much clearance to function perfectly. But the rounded magazine contour does not as readily accommodate flat nose bullets such as hollow point and truncated designs. If the front of the magazine was square, then the bullet shape would not matter. But it’s not square, it’s round. Remember the saying about not being able to fit a square peg in a round hole? That applies here. Therefore, flat nose bullets must be seated deeper in order to fit in the round profile of the magazine. This is illustrated in the Figure 1. Part A shows the nose profiles of a round nose and flat nose bullet of the same length. The front profile of the flat nose bullet is much wider than the round nose design. B shows how the bullets would fit in a single column magazine if loaded to the same maximum length. The flat nose bullet would not fit but the round nose bullet would. C shows that the flat nose bullet must be seated deeper in order to fit in the magazine with the same clearance as a round nose bullet.
Do flat nose bullets rub on the round edge of a double column magazine? I’ve looked at my double column Para Ordnance magazines, but it’s a bit tough to tell without cutting a “window” in the magazine that is larger than the notch for the magazine catch. From what can be seen through the magazine catch notch, the edge of some flat nose bullets appear to be right at the edge of where the front face of the magazine starts to curve. This suggests that there might be some contact there. Even if there is contact between the side of the bullet nose and the edge of the magazine, it might not cause the cartridge to jam. Because this is a double column magazine, the nose of the bullet would touch only one side of the magazine. The nose could get pushed slightly toward the center of the magazine. This might or might not impede function.
#2. Cartridges sit in the magazine at an angle. Figure 2 shows a side view of the magazine. The cartridge is canted, and lies at an angle of 68 degrees (in the 1911 single column magazines that were measured). (It turns out that the angle changes as more cartridges are added, and this is explained in more detail below). Note also that only an edge of the cartridge rim contacts the back wall. Because the cartridge is at an angle, you have to take the diagonal length of the cartridge into account because it’s this length that has to fit in the magazine.
Because cartridges sit at an angle, the nose of flat nose bullets acquires a somewhat rounded profile, and they don't look exactly like is shown in Figure 1. But the bullet's angle does not transform it to a simple round nose like a real round nose bullet. This is illustrated in Figure 3. You'll note that the edges of the flat nose still protrude beyond the profile of a normal round nose design.
The diagonal length of a cartridge loaded with flat nose bullets is longer than a cartridge loaded with round nose bullets when they are seated to the same depth. This is illustrated in Figure 4.
Round Nose Bullets. A standard 130gr factory FMJRN bullet that is seated to the SAAMI maximum overall length of 1.280 will gain about 0.015 inches when measured diagonally. That adds up to1.295 inches. Some 38 Super round nose bullets do not have the same profile. As noted in the Factory Ammunition section of this website, Remington, Winchester, Fiocchi and Aguila RN bullets have a rounder nose than the somewhat pointier Federal, Magtech, and American Ammunition bullets. The more pointed round nose bullet, gains less length, roughly only 0.014 inches when measured diagonally.
Flat Nose Bullets. Cartridges with flat point bullets gain between 0.027 and 0.031 inches in diagonal length. A 38 Super round loaded to 1.280 inches with a flat nose bullet has an overall diagonal length between 1.307 and 1.311 inches.
Bullets with a broader flat nose profile have a longer diagonal length. That’s because the edge of the flat nose is farther from the midline of the bullet. You mathematical types can refer to Pythagorean geometry to see why that's the case. (The wider the flat nose, the longer one leg of a right triangle becomes, which translates into a longer hypotenuse.)
Table 2 lists some common bullets and the diagonal length of the cartridge when the bullets are loaded to an overall length of 1.280 inches. Note the diagonal length of the two Hornady XTP bullets listed. The flat nose profile of the 147 grain XTP bullet is broader than that of the 124 grain bullet. This results in a longer diagonal measurement for the 147 grain bullet.
Table 2: Diagonal Length with Round Nose and Flat Nose Bullets
* Length in inches. ** Some hollow point noses have a non-circular geometric shape which results in variable diagonal length measurements depending on whether the measurement is made at the geometric point or between points. Measurements made with standard semi-rimmed 38 Super brass. With rimless brass the diagonal length increased about .003 inches less. Weight in grains. FMJ = Full Metal Jacket. GDHP = Gold Dot Hollow Point. GS BJHP = Golden Saber Brass Jacketed Hollow Point. JHP = Jacketed Hollow Point. STHP = Silver Tip Hollow Point. XTP HP = Extreme Terminal Performance Hollow Point.
I empirically tested the nose profile concept. This test compared two different flat nose hollow point bullets, one with a narrow flat profile, and the other with a wide flat profile. I used a Winchester Silver Tip Hollow Point (STHP) for the narrow flat nose bullet, and a Sierra Jacketed Hollow Point (JHP) for a wide flat nose bullet (refer to Table 2 above). For this test, I loaded these bullets to an overall length that is much longer than they should be loaded. The bullets were loaded to different overall lengths. The STHP was loaded to a longer than SAAMI maximum (1.280) length of 1.285 inches and had a measured diagonal length of 1.312 inches. The Sierra bullet was seated deeper, right at the SAAMI maximum length of 1.280 inches, and had a diagonal length of 1.311 inches, which was 0.001 inches less than the STHP cartridge. These were tested in the same Colt magazine. I put the test cartridge in as the second round, then added more rounds. The first round, and the rounds put in after the test cartridge were Winchester factory hardball (FMJ) that were tested and fit the magazine with no problems. The average length of the Winchester rounds used in this test was 1.273 inches and the longest one measured 1.279 inches.
The important information here is that the narrow Winchester STHP bullet was seated to an overall length longer than the wide Sierra JHP bullet. All else being equal you would predict that the longer seated STHP bullet would jam in the magazine while the shorter seated Sierra bullet would not. Right? But that wouldn’t make an interesting example, would it?
Result: The result was that the shorter seated Sierra JHP bullet got stuck in the magazine, and the longer seated Winchester STHP bullet did not. After I put two rounds in on top of the Sierra test round, it jammed. The two top rounds then fell out of the magazine and the Sierra round was stuck tight. It had to be coaxed out. The narrower (longer) Silver Tip Hollow Point bullet did not jam at all and I was able to fill the magazine completely and unload it normally. I repeated the test several times and the results were the same.
This simple test demonstrates that wider flat nose bullets have to be seated deeper in order to accommodate the rounded profile of the front of the magazine, which is what Figures 1 and 3 (above) are all about.
Another thing I learned during this test was that there are (or can be) slight differences in the front to back width at different depths of the magazine. When I tested the Sierra bullet it jammed solid in the upper portion of the magazine. But if I forced more rounds in, pushing it deeper, it started to move freely as per normal. This was not due to pushing the bullet deeper in the case because I measured it afterwards and the cartridge's length had not changed.
Cartridge Fit in Magazine
Now that the basics are out of the way, I'm going to toss a curve ball at you. The fact of the matter is that the angle at which the cartridge lies in the magazine changes as a function of how many cartridges are in the magazine. Cartridge length, related to bullet shape, has to be flexible enough to fit at these changing angles. The information that follows is an attempt to explain this in more detail.
A simple way to look at this is that the cartridge length cannot be greater than the shortest distance in the magazine. The shortest distance in the magazine is a straight line that intersects the front and back sides of the magazine at a 90 degree right angle (see Figure 5). The longest length of the cartridge is measured at a diagonal angle as noted above. The question is whether the longest length of the cartridge (diagonal angle) ever crosses the 90 degree angle of the magazine? After all, if the cartridge always stays at a sufficiently steep angle, it might never approach the angle of the shortest distance.
We need to know 2 things. What is the angle of the cartridges in the magazine, and what is the angle of the maximum diagonal length of the cartridge? If you add these numbers and they total 90 degrees or more, then the cartridge’s longest length matches, or crosses, the angle of the shortest distance.
Angle of Cartridge in the Magazine
I noted above that cartridges lie in the magazine at a 68 degree angle. That’s true for the top round. Oh, and at least for the time being I’m only talking about 38 Super rounds in a single column magazine. Okay, well the 68 degree angle only applies to the top round, and it only applies when there is no more than a round or two in the magazine. As more rounds are added, the angle of the top round changes. Strange, but true. By the time a 10 round magazine is full, the angle of the top round lies at 72 degrees – a change of 4 degrees.
But what is more unusual is that the angle of the rounds deeper in the magazine changes even more than the angle of the top round. When a 10 round single column magazine is full, the angle of the rounds under the top round is 78 degrees. This is a change of 10 degrees! The measurements are listed in Table 3.
Table 3: Angle of Cartridges in the Magazine
Angle is relative to the back wall of the magazine as shown in Figures 2 and 6. Values represent the average angle from 7 different magazines. Magazines were Chip McCormick with factory follower and Colt with Wilson Combat follower. The brand of follower did not influence angle. NA = Not Applicable: Angle of underlying round was the same as the top round.
This difference in the angle of the top round and the one immediately under it causes a substantial gap at the front of these cartridges. Look closely and you can see this in Figure 6. This gap is one of the principle causes of nose dive feed failures which is discussed in more detail here (under construction) on this web site.
Angle of Longest Length
So, at what angle is the loaded cartridge at its longest length? This is where I turn to math. Remember when your teachers said you would need math skills one day? This is the day. But I am going to cheat and use a calculator. I measured the width of the flat surface of hollow point bullets and derived the numbers necessary to plug into a program on the web (angle calculator) to determine the other angles of a 90 degree right triangle. Let's use some numbers from the recommended overall lengths in Table 1. The most extreme angle will result with the Sierra JHP because it has the widest flat nose (roughly .230 inches in diameter) and usually needs to be seated at the deepest depth. If it is loaded to an overall length of 1.190 inches, its angle of longest length is 15 degrees (actually 14.96, but who's counting?). Using the rather narrow flat nosed Winchester STHP bullet (roughly .200 inch diameter nose) seated to 1.250 inches produces a 13.62 degree angle of longest length. In fact, if you used a sharp pointed bullet (zero width tip) the angle of longest length (loaded to 1.280 inches) is 9 degrees. That's the smallest possible angle. The round nose bullets probably have a longest angle of around 10-12 degrees.
If I add the Sierra's 15 degree angle to the 78 degree angle of deep cartridges in a fully loaded magazine, the combined angle equals 93 degrees. Even the narrow nosed Winchester STHP has a combined angle of 91.62 degrees. This means that the angle of the longest length of the cartridge with a flat nose bullet crosses the critical 90 degree angle of the shortest distance. This is shown in Figure 6. In fact, if you add the angle of round nose bullets (estimated 10 to 12 degrees) to the 78 degree cartridge angle and you're close to or at 90 degrees anyway. So pretty much any bullet configuration will need to fit in the shortest distance in the magazine.
This means that the maximum diagonal length as measured here is relevant in determining the maximum length that will fit in the magazine. Thank goodness. I’d hate to have done all this work for nothing.
Bullet shape and chambering
The shape of the bullet nose determines how deeply the bullet must be seated in order to fit in the chamber without engaging the riflings. Bullets with short noses must be seated deeper than bullets with long noses. This is illustrated in Figure 7.
If the bullet engages the riflings before the cartridge has fully chambered, the slide might not close, which can prevent the gun from firing, or in extreme cases might contribute to the gun firing "out of battery" which can be bad for the pistol, and the shooter and bystanders if the case ruptures.
Different bullets may look very much alike in their shape, but differences in where the shoulder starts can yield very different "just fit" seating depths. Here is a good example. Figure 8 shows two round nose FMJ bullets, a Sierra 125 grain FMJ and a Hornady 124 grain FMJ. They have the same shape, but not the exact same shape. The Sierra bullet has a shorter nose than the Hornady bullet. (The nose is defined as the part of the bullet forward of the shoulder. The shoulder is the transition from the bullet shank (driving band) to the nose.) The blue bar to the side of each bullet represents the length of the bullet's nose. The shorter nose of the Sierra bullet means that it has to be seated deeper than the Hornady bullet when both are loaded to their maximum "just fit" length. Tests in one barrel showed that the Sierra bullet had to be seated 0.068 inches deeper than the Hornady bullet to "just fit" in the chamber. That's a big difference.
Another factor that can influence overall length is the shape of the reamer that was used to cut the chamber. One reamer might have a conical shape for the leade. Another reamer might have a rounded shape for the leade. The shape of the bullet's shoulder will determine how it interacts with a particular barrel's leade angle/shape. If you load bullets close to their maximum "just fit" length then you'll need to test every specific bullet design to determine that length.
If you're loading for several barrels, you will need to determine which barrel has the shortest throat and use that measurement for all your ammo. Or you can be crazy and have different overall lengths for different barrels. Overall length can be critical to accuracy in rifle barrels, but it appears that it is less critical for pistol barrels, so loading to a single overall length to fit all your barrels usually does not mean sacrificing a great deal of accuracy in barrels with longer throats. Of course, I'm sure that someone will write me and challenge that claim, but there you have it.
To test the maximum length that the bullet can be seated to, load a cartridge with the bullet seated long. Remove the barrel from your pistol. Drop the loaded cartridge into the chamber (make sure the chamber is clean). If the bullet is seated too long it will engage the riflings and will not fully chamber and/or you'll be able to feel it "stick" in the chamber as you try to remove it. You might even see marks on the bullet where it contacted the riflings. Seat the bullet a little deeper and try it in the chamber again. If you adjust in small increments you can determine the length that will "just fit" in the chamber without touching the riflings. But in practice, you'll need to seat the bullet deeper than this since the overall length of individual rounds can vary during your loading procedure (for a variety of reasons). And minute differences between bullets means that some will touch the riflings while others won't, even when they are seated to the exact same overall length. You need a margin of error to compensate for differences in actual overall length and minute differences in bullet shape. I tend to set my dies to seat the bullet to at least 0.010 to 0.015 inches deeper than the "just fit" length.
Overall length and bullet nose shape interact to regulate feed timing. Feed timing refers to the position of the slide as it moves forward to chamber a round. Proper timing requires the cartridge to be at a particular stage of the feeding process relative to slide position. If it isn’t, malfunctions can occur. There is a range of overall lengths over which a given bullet will meet this timing requirement, and sensitivity to this can vary with gun design.
Feeding cartridges change angle twice. First when the bullet nose hits the feed ramp and they angle upward, and second when the bullet nose hits the roof of the chamber, the tip-over point, and the cartridge angles downward to straighten out and enter the chamber. During this latter stage the rear of the cartridge slides up the breech face and the rim slides under the extractor hook.
The shape of the bullet nose and cartridge overall length affect the precise time when these events occur during the round’s forward movement. These features also affect cartridge angle at the tip-over point. Cartridge angle is critical here because if the tip-over angle is too steep the round won’t straighten out and enter the chamber. Short cartridges and some flat nose bullet profiles will result in a steeper tip-over angle. Here is an example. I experienced feeding malfunctions in a CZ 75 B 9mm pistol with ammunition that was loaded for a Para Ordnance 9mm pistol whose barrel had a very short throat, requiring the rounds to have an unusually short overall length. The Para Ordnance fed the ammunition fine, but the CZ choked. Two types of malfunctions occurred. In the first, the round would stop at the tip-over point. In the second, the round would end up vertically oriented with the bullet nose under the hood. I saw this as an opportunity to test overall length on feeding reliability.
To understand how short overall length rounds cause feeding malfunctions, see the cartridges in Figure 9A. The black round is a 9mm Luger with a round nose bullet loaded to 1.122” overall length. They feed reliably and smoothly in the CZ. The red round has the same bullet, but it’s seated much deeper at 1.022” overall length. This round produced a dismal 38% failure rate in the CZ. Figure 9B shows that if the slide is farther forward at the tip-over point because of an especially short cartridge, the cartridge’s angle is much steeper.
With a longer overall length, the cartridge rim slides up the breech face as the slide moves forward (top of Figure 9C). The long round’s straightening path (dark gray) is gradual and sloping. But with a short overall length, the cartridge rim must slide up the breech face with very little forward movement of the slide (bottom of Figure 9C). The short round’s straightening path (pink) is nearly vertical during its initial movement (see comparison in Figure 9D). The pushing force of the slide can jam the short round in the tip-over position or shove the cartridge into a vertical orientation as the rim slides down the breech face instead of up (bottom of Figure 9C).
It’s worth noting that some other feeding problems can look like those illustrated here for short overall length but are not caused by short overall length.
Speer Reloading Manual #13. 1998. Ed. Allan Jones. Blount, Inc. Sporting Equipment Division. Speer, Lewiston, ID.
ANSI/SAAMI booklet Z299.3-1993. American National Standard. Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Pistol and Revolver Ammunition for the Use of Commercial Manufacturers. 1993. Sporting Arms & Ammunition Manufacturers' Institute, Inc., Wilton, Conn. USA.
1-6-13 Added Feed Timing section.
|Questions, comments, suggestions, hate mail? Feel free to email me. However, the probability of getting a response is low simply because I have a day job and a life and don't have the time to respond to all emails. It's nothing personal, really. Nevertheless, I do appreciate your thoughts. If you see an obvious error then please put the word ERROR in the title of your email. Thanks, and happy shooting.|
Copyright 2006 - 2013 38 Super.net