• Short Rifle Barrel Accuracy
• How Far Apart Are Rifle Barrel Accuracy Nodes In The Body
• How Far Apart Are Rifle Barrel Accuracy Nodes In One
• How Far Apart Are Rifle Barrel Accuracy Nodes Made
• How Far Apart Are Rifle Barrel Accuracy Nodes Located
• How Far Apart Are Rifle Barrel Accuracy Nodes In Back

The 'whip' theory suggests a barrel's thickness and length impacts how much it moves with each shot. Indeed, a short thick barrel mounted nearly in-line with the stock butt (as used on bench rest rifles) should produce far less 'whip' than a field profile long hunting barrel with a high cheek piece. Zero the rifle at 100 and put a large target out at 400-600 yards. Load the charges that you want to test from.2 to.5 grains apart. Fire 3-5 shots of each and note where they land on the target. Some will result in a vertical difference on the target. Some will group together. The ones that group together are your accuracy nodes. The lands of the rifle are where the rifling of the barrel starts. If you have a bolt action rifle, you can remove the bolt and look from the rear of the rifle out the front of the barrel. You will see some spirals in the rifle barrel. The spirals give the bullet spin and help it to be more accurate. What is the bullet ogive?

Have you been wondering how to find the bullet seating depth for best accuracy in your rifle? In this article I’ll go over how you can fine-tune the bullet seating depth to achieve even greater accuracy. Hopefully by the end you’ll be able to take this knowledge and tighten up those groups.

The way to achieve the bullet seating depth for best accuracy in your rifle is to determine how far off of the lands of your rifle to seat the ogive of the bullet.

What are the lands of a rifle?

The lands of the rifle are where the rifling of the barrel starts. If you have a bolt action rifle, you can remove the bolt and look from the rear of the rifle out the front of the barrel. You will see some spirals in the rifle barrel. The spirals give the bullet spin and help it to be more accurate.

What is the bullet ogive?

The bullet ogive pronounced “oh-guy-ve” is the front section of the bullet where it starts to curve towards a point. You can see where this is in the below picture.

Why is the bullet ogive so important?

Short Rifle Barrel Accuracy

In the course of reloading you will notice while looking at reloading recipes that the COAL often gets mentioned. The COAL is the cartridge overall length. It is measured from the base of the cartridge to the tip of the bullet. This measurement is important to make sure that your cartridge will load properly into a magazine. However, length of individual bullets can vary. For example, I took 6 different rifle bullets from the same batch and measured them. Below are the lengths I found:

• 1.4035″
• 1.403″
• 1.400″
• 1.405″
• 1.403″

Try it with a random sampling of your own bullets.

Because the length of the bullet can vary slightly across all bullets, there is another measurement that is helpful in order to obtain the bullet seating depth for best accuracy for the bullet you are shooting. This measurement is known as the CBTO.

What is CBTO?

CBTO is the Case Base to Bullet Ogive. Your bullet length may have variations. The ogive is going to have much less variation. This means that we can use it to determine a more consistent seating depth for the bullet.

Using Ogive to Determine the Bullet Seating Depth For Best Accuracy

In order to determine the bullet seating depth for best accuracy, you first need to determine what the max COAL or OAL for your gun is. There are a variety of ways to do this.

How Far Apart Are Rifle Barrel Accuracy Nodes In The Body

OAL Length Gauge

Hornady makes an OAL length gauge. You can find it on Amazon here: Hornady OAL Gauge. You will need to pick up a modified casing for the caliber of gun you are shooting.

This gauge is used by attaching the modified case to a rod and then inserting one your bullets into the case. The case is then inserted into the rear of the rifle and the bullet is pushed forward until it barely hits the lands.

If using this method it is important to measure using the exact same bullet every time. You may notice overtime as you shoot your rifle more more the lands will wear down and retreat somewhat. This is because the more you shoot the rifle the more the explosive energy of the cartridge starts to wear away at the rifling at the mouth of the barrel.

As there can be some variation in bullets you want to make sure you use the same bullet every time if you are using this method. If you are shooting a .270 and shoot bullets from two different companies, you will want to do the same for each brand. Each brand will have a slight variation on their bullet. Keep one bullet from each brand to reuse each time you measure the OAL for your rifle.

One thing you will want to do is make sure the base of the shell to the tip of a trimmed casing that has been fired from your is the same as the modified bullet in Hornady OAL kit. If it is not, you will need to adjust your calculations to take this into account.

In the process of using the gauge you, may find that the bullet becomes stuck in the barrel of the gun. You can use either a cleaning rod to gently tap out the bullet or place the gun with the butt on the ground and gently tap it. Either way should cause the bullet to come free.

Curved OAL Gauge

If the gun you’re using does not allow the bolt to be removed or there is now way to get rear access to the chamber, then you can use a different version from Hornady that is curved.

Lower Cost OAL Gauge

Frankford Arsenal makes a lower-cost version of an OAL gauge however this one is made of plastic: Frankford Arsenal OAL Gauge. It also costs less than the Hornady version.

I figured out the OAL of my gun, now what?

Now that you know what the max OAL of your gun is, you can start to adjust bullet seating depth either closer into the lands or away from the lands.

Two terms you will want to be familiar with when adjusting the bullet seating depth are jam and jump. These relate to whether the bullet is seated further into the rifling or away from the rifling.

• Jam – A jammed bullet is a bullet inserted further into the rifling of the barrel beyond the initial point where it touches the rifling. You can remember this by thinking of the bullet being jammed further into the rifle barrel. You need to be extremely careful when using a jammed bullet because this can cause greater pressure than what normally would be present. Also, if you are hunting or in a situation where you want to remove a cartridge from the chamber without firing it, a jammed bullet may stay stuck in the barrel. The bolt could pull the casing free from the bullet.
• Jump – A jumped bullet starts further back from the lands of the rifle barrel. One way to remember the meaning of a jumped bullet is to think that the bullet has to jump a little bit before it reaches the opening of the rifle barrel.

Adjusting the Seating Depth

Once you figure out where the lands are in relation to the ogive of your bullet, you will most likely want to start 0.015” to 0.020” back from the lands. At this point you can start to adjust the seating depth of the bullet and notice how it affects the accuracy and grouping of your rounds.

After shooting a group and examining the accuracy, you can increase or subtract 0.005″ and go from there. If you are putting more of a jam on the bullet make sure that you are using a lighter load as a jammed bullet causes higher pressure in the chamber. Keep adjusting the seating until you find the bullet seating depth for best accuracy in your rifle.

Using a Bullet Comparator

In order to measure where the ogive should be in relation to the lands, you will need a tool known as a bullet comparator. Hornady also makes one of these: Hornady Comparator. It comes with a variety of gauges for different diameter bullets. The previous link comes with a set of 14 inserts for measuring different calibers. It will measure from 17 to 45 caliber. Another nice accessory to have is the anvil base kit from Hornady: Hornady Anvil Base. This attaches to the other arm of your calipers and provides a wider base for the casing to rest on when measuring the ogive.

After determining your CBTO based on how far away from the lands your bullet ogive should be, you can set up your press to seat the bullet. Then use the comparator tool to measure and make sure the cartridge base to ogive(CBTO) is where you are expecting it to be.

One thing to also consider is that you want to leave at least the width of the bullet in the casing. So for instance if you are shooting a .284″ diameter bullet, leave at least .284″ of the base of the bullet inside of the shell casing. This will help to stabilize and seat the bullet.

Quick Recap For Obtaining the Bullet Seating Depth for Best Accuracy

1. Find the OAL for your rifle using an OAL gauge. Then you will know where the lands start and then you can start to play with the CBTO for your rifle chamber. Make sure that if you are using Hornady’s kit, that you measure their modified casing and a trimmed brass casing shot from your rifle. There may be some difference here.
2. Seat the bullet with the ogive starting at 0.020″ from the lands or rifling of the barrel
3. Start increasing or decreasing 0.005 in at a time until you achieve the grouping you desire. Be careful if you are putting a jam on the bullet as this will caused increased pressure. You don’t want to start with a max powder weight if you are using a jammed bullet.
4. In order to measure the CBTO, use a bullet comparator tool
5. Have at least the width of the bullet inside the casing. So if you are shooting a .284” diameter bullet, keep at least .284” of the bullet seated inside the shell.

Further Reading

In the course of researching this article I found a couple of sources that you may want to read for some additional knowledge.

• The first is an article on the Nosler website. It gives a way to measure bullet seating depth using a marker and a bullet. This can be used in place of the overall length gauge although it may not be quite as accurate and you’ll want to take several measurements as it recommends to get an average. You can find the article here: Nosler Article
• The second is an article on bergerbullets.com. It has a great discussion of COAL vs CBTO. It also discusses why you can’t just tell your buddy the CBTO you are using and expect him to measure the same with his equipment. You can find the article here: Effects of Cartridge Over All Length (COAL) and Cartridge Base to Ogive(CBTO)

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Barrel Tuner Vibration Analysis:
Effects of tuner adjustments on vibration frequency
and the shift in barrel node points.

This analysis was done with the LS-DYNAFinite Element Code. This software is a very powerful tool for analyzing thedynamic and static loading of structures. It was used here to calculate theVibration Mode shapes and Natural Frequencies of a rifle barrel with a tunerinstalled and with the tuner positioned at slight different positions. Thebreach end of the barrel was fixed in space which treats the barrel as acantilever beam. There is a vertical plane of symmetry in the calculation andonly motions in the vertical plane are calculated. The displacements are greatlyamplified so that they can be viewed.

THE BARREL.... The 416 Stainless Steel barrel in this calculation hasa breach diameter of 1.5' with a constant diameter for 4' and then astraight taper to a diameter of 0.8' at a distance of 20'. Itsdiameter is 0.8' from there to the end of the barrel at 22'. Thecaliber is 6mm with no rifling modeled. The 416 Stainless Steel tuner has an0.8' ID and the OD is 1.5' and extends 2' beyond the muzzle inthe initial position. This barrel weighs 6.61 pounds and the tuner weighs 1.11pounds. Two additional calculations were done with the tuner moved 0.1' tothe right and 0.2' to the right (farther from the breach). The breach endof the barrel is fixed to simulate the barrel's attachment to the rifle'saction. For ease of generating the 3-D model, I did not include the cartridgechamber or the thread ligament.

How Far Apart Are Rifle Barrel Accuracy Nodes In One

HOW ACCURATE ARE THE CALCULATIONS?.... I have run a couple of testcases comparing the FEA results with classical methods of analysis for modeshapes and frequencies. The deflection (sag) calculated with FEA is checked foraccuracy. Accuracy Comparison.

Mode 1 - 80.304 Hz

Mode 2 - 439.22 Hz

ADDING A TUNER.... Adding a tuner to the muzzle of a rifle barrel doesthe following:
1. The additional mass reduces the amplitude of the vibrations.
2. Decreases the natural frequencies by decreasing the lower Mode's frequenciesmore than the higher Modes.
3. Increases the barrel's vertical end sag due to the extra weight. This wouldtend to make the vertical plane the preferred plane of vibration.
4. Moves the Mode 2 node closer to the muzzle.

POSSIBLY MODESHAPES ARE NOT SO IMPORTANT.... I ran some FEA calculations of how a barrelreacted to the high gas pressure and recoil. The forced deformations from thehigh pressure gas and recoil cause the muzzle to change where it is pointing atthe target when the bullet exits the muzzlet. Click on the graphic to read theinfo on Esten's rifle. Esten did extensive testing so that the FEA model couldbe normalized to his test data.

Click here or on the graphic to view the calculationresults.

CONCLUSION.... Maybe the 'consensus' was that a rifle barrel vibrated in one or more of the mode shapes when fired. That was because the mode shapes and frequencies were easy to calculate and they did seem to answer some of the questions. From these FEA dynamic pressure calculations, it appears that the recoil and forced deformations are much more important than the natural vibration modes in determining where a barrel is pointing when the bullet exits the muzzle. Then after the bullet exits the muzzle, the rifle barrel vibrates in its various natural frequencies and mode shapes. Put another way, consider a guitar string being plucked. One pulls the string into a position (forced position) then releases it and the string vibrates at is natural frequency. The recoil and bullet motions 'pulls' the rifle barrel to a new shape and once the bullet leaves the barrel, then the barrel vibrates. However, the addition of the scope to the model has shown some small high frequency vibrations superimposed on the forced deformations, both of which, slightly alter where the muzzle points before the bullet exits. For lowering the amplitude of the high frequency vibrations, it appears that even an 'out of tune' tuner is better than no tuner at all.

How Far Apart Are Rifle Barrel Accuracy Nodes Made

COMPLEX VIBRATION.... The top animated gif is Mode 1 of the barrelwith a tuner and the Mode 1 vibration frequency is 80.304 Hz. The bottom view isMode 2 vibration that has a frequency of 349.22 Hz. The ratio of the frequencyfor Mode 2 to Mode 1 is approximately 4.34 to 1.0. I have tried to make theanimations with that same frequency ratio so the viewer can visualize thecomplexity of what is occurring with the barrel and especially the displacementof the muzzle and the angle of departure of the bullet. In your mind's eyeimagine the two modes of vibration going on at the same time with the amplitudeof Mode 1 larger than Mode 2. As the larger swings of Mode 1 are occurring thesmaller but faster amplitude swings of Mode 2 are superimposed on Mode 1.

ADJUSTING THE TUNER.... As the tuner is adjusted by moving it fartherto the right on the barrel (farther from the breach), the frequencies of bothmodes decreases. Here is a table of the resulting Mode frequencies as the muzzletuner is located at the original position and then moved to the right 0.1'and then 0.2'. A separate calculation was done with the tuner removed as abase condition which has a much higher frequency in both Mode 1 and Mode 2 but alower frequency ratio of Mode 2 to Mode 1 of 3.804. Not only does the tunerlower the frequencies, but it changes the ratio between the two frequencies.

Table 1: Mode Frequencies

Tuner Position (in)	Mode 1 (Hz)	Mode 2 (Hz)	Mode 3 (Hz)	Ratio Mode 2/Mode 1
Original Position	80.304	349.22	919.47	4.349
0.1 Shifted Right	79.568	345.77	910.22	4.346
0.2 Shifted Right	78.837	342.38	901.13	4.343
None	130.91	498.00	1200.1	3.804

How Far Apart Are Rifle Barrel Accuracy Nodes Located

MODE 2 NODE LOCATION.... For the tuner in the original position, thenode where the barrel's center line stays on the original axis is 1.60'back toward the breach from the muzzle. You can view the location in theanimated gif picture. Find the node location by putting your mouse pointer onthe center of the bore and move it to the right or left until the verticalmotion of the center of the bore remains fixed behind the mouse pointer. Thetuner mass would have to be moved much farther away from the breach to cause thenode to be located at the muzzle.

How Far Apart Are Rifle Barrel Accuracy Nodes In Back

BARREL SLOPE AND CURVATURE.... In this view, I placed a dotapproximately at the node point. At the muzzle upward swing of Mode 2, the slope(first derivative of displacement) of the barrel is positive on both sides ofthe node. The change in slope or curvature (second derivative of displacement)is also positive on both sides of the node. The bore is not parallel to the axisat the node location. The only time the bore is parallel to the original axis iswhen the barrel swings through the rest position. At that time, of course, everypoint on the barrel is parallel to the axis. The same is true for Modes greaterthan Mode 2, at each central node. The bore is always parallel to the axis atthe anti-nodes (minimum and maximum) points between the nodes.

SIMPLY WRONG.... There is an article on the web from the PrecisionShooting Magazine that is wrong about how a barrel vibrates. Here is thearticle: I'mFeeling Those Good Vibrations, AGAIN! A barrel tuner can improve accuracybut this article is wrong about how a barrel vibrates and how tuners work.

EXIT ANGLE.... If the tuner were long enough to cause the node of Mode2 to be located at the muzzle, the muzzle would still have the variations inexit angle and the exit angle would not be parallel to the original bore axis,except when the barrel passes through the rest position twice on each cycle.

BARREL SLOPE WITH NO TUNER.... The dot is approximately at the nodepoint. Zooming in at the Mode 2 node while at the muzzle's maximum upward swing,the upward slope and positive curvature is more obvious in the barrel withoutthe tuner.

Table 2: Mode 2 Node Location

Tuner Position (in)	Node Distance from Breach (in)	Node Distance from Muzzle (in)	Muzzle Sag (in)
Original Position	20.40	1.60	0.00233
0.1 Shifted Right	20.45	1.55	0.00237
0.2 Shifted Right	20.58	1.42	0.00241
None	16.95	5.05	0.00106

Mode 1 -130.91 Hz

Mode 2 -498.00 Hz

TUNER REMOVED.... For a base condition, here are thefirst two vibration Modes for the same barrel with the Tuner removed. The barrelvibrates with approximately a 38% higher frequency with the Tuner removed. Thevibration frequency of Mode 2 is slightly less than four times that of Mode 1.The Mode 2 node is much farther back from the muzzle.

Possible explanation of howa tuner could improve accuracy

MUZZLE POSITION.... The Tuner lowers the frequencies ofeach Mode so that the muzzle could be just approaching the upper extreme of itsswing at the time the bullet exits. The Tuner also adds weight to the barrel'smuzzle which increases the vertical deflection (sag). More sag before firingwould make the barrel more likely to vibrate in the vertical direction.

MUZZLE EXIT ANGLE.... Tuner changes the frequency of eachmode when it is adjusted and not at the same rate. The exit angle is probablymore important to shot placement than the vertical position of the muzzle. TheTuner could be adjusting the muzzle exit angle so that when the deflections ofMode 1 points up, the Mode 2 is pointing down. This could be happening at thetop or bottom of the Mode 1 muzzle swing.