Why does seating depth change accuracy?

16 November 2003, 13:29

I know that seating depth does indeed change accuracy, but why it exactly does is not clear to me. Can someone help me out?

I have read that a change in seating depth can increase or decrease the amount of time from ignition to muzzle exit, and this can work with or against the vibrations that a barrel undergoes during firing. This seems somewhat plausible to me.

I have also read the less bullet jump, the greater the chance that the bullet enters the rifling square. So far, this is very unconvincing to me because I just cannot imagine the that point of entry makes a significant difference when the bullet will be travelling through 24" of rifling immediately after.

Does anyone have a good explanation of why various seating depths affect accuracy?

Thanks!

16 November 2003, 14:20

CMcDermott

You have the two reasons. It seems you don't believe the second one. If a bullet doesn't enter the rifling square it doesn't "square up", it deforms instead; just too much pressure behind it. And deformed bullets, especially ones that don't deform the exact same way each time, don't fly to the same impact point on the target. Remember - accuracy is just the EXACT same thing happening time after time, and anything that changes, like the bullet entering the rifling tipped a few degrees differently than last time, will probably have a negative impact on the accuracy.

16 November 2003, 14:36

djl4570

If the bullet gets slightly cockeyed going into the rifleing, it's going to come out slightly cockeyed at the muzzle. This will cause it to wobble in flight because the axis of rotation is different from the centerline of the bullet. A bullet that swages squarely onto the rifleing will rotate around its centerline and fly straight. Changing the seating depth also changes the loading density of the cartridge. This can change the way a load performs in your rifle although not to the same degree as reducing freebore.

I recently did a test in a .308 Sako Tactical I have had for years. The rifle has a long freebore do I did an experiment where I seated bullets .025 increments starting at 2.800" until I was within .020" of the lands. I shot four five round groups of each using the same charge of powder and found that the closer the bullet was seated to the lands the more accurately the rifle shot. I'll have to dig out the targets for specific details.

I am ready to rebarrel this rifle to correct the excessive freebore, loose neck and sloppy chamber. It's the only bolt action .308 I have that ejects brass that will not chamber in either of my match grade M1A rifles.

16 November 2003, 16:11

Clark

I have a friend who has a rifle that is most accurate with the bullet jammed into the lands.

He was hunting, and at lunch, he pulled the round out. But the bullet stayed in the lands and the case spilled powder all over in his rifle.

Not having a cleaning rod, he had to use someone else's rifle with the bullet and powder removed, shot muzzle to muzzle to get his bullet to pop out.

Now he carries a cleaning rod.

17 November 2003, 04:42

243winxb

quote:
Originally posted by FixedPower:
I know that seating depth does indeed change accuracy, but why it exactly does is not clear to me. Can someone help me out?

I have read that a change in seating depth can increase or decrease the amount of time from ignition to muzzle exit, and this can work with or against the vibrations that a barrel undergoes during firing. This seems somewhat plausible to me.

I have also read the less bullet jump, the greater the chance that the bullet enters the rifling square. So far, this is very unconvincing to me because I just cannot imagine the that point of entry makes a significant difference when the bullet will be travelling through 24" of rifling immediately after.

Does anyone have a good explanation of why various seating depths affect accuracy?

Thanks!

Seating depth changes the pressure. Some powders burn better with higher pressure. AS for less bullet jump, it is not that important. In hunting guns the round has to be centered in the chamber for best accuracy. To center the round you can do it 3 ways. 1.Have the bullet jammed into the rifling. 2.Use a neck sizing bushing die and only size part of the neck, the unsized neck area will conform to the chamber and center the round. 3. Rebarrel the gun with a tight neck chamber.

17 November 2003, 05:07

NotRicochet

Obviously, if a long freebore is reamed very close to the bullet diameter, tipping of the bullet isn't going to be a major factor. It is if the freebore is significantly larger than the bullet. I still can't believe there's a lot of difference even with generously sized throats in how the bullet tips over varying distances much smaller than, say, half the bullet diameter. Close fitting case necks in the chamber will obviously guide the bullet better. Seems to me the main issue is keeping the bullet aligned both concentrically with and parallel to the bore, and that's mostly an issue of cartridge alignment in loading and close fit of the case to the chamber to keep iit from sagging off to one side. That's where seating right to the lands gives a point of support at the front and the tapered leade of the rifling guides the bullet in straight, the velocity and pressure being too low to deform anything. But anything short of the rifling origin shouldn't be much help.

17 November 2003, 12:47

Hot Core

Hey Fixedpower, First off Welcome Aboard. Always good to see another new member.

Since you have other folks hammering away at the second part of your post, I'll help you out with the first part.

quote:
Originally posted by FixedPower:
I know that seating depth does indeed change accuracy, but why it exactly does is not clear to me. Can someone help me out?...

Basically all you need to know is that when the Bullet begins moving down the Barrel, your muzzle begins moving in an elongated Figure " 8 " Harmonic. If you could look at the muzzle(without catching a bullet) you might notice the " 8 " may be skewed slightly to the left or to the right.

When you adjust the Seating Depth, it changes the point where the Bullet Exits during the Harmonic. You want the Bullet to Exit the Barrel when the Bullet is either at the Apogee or Perigee because the Barrel has "almost stopped moving" and is getting ready to begin going the other way.

This is basically what makes the Browning BOSS work, because it allows the shooter to adjust the "Harmonic" on Factory Ammo (or Reloads for that matter) without having to adjust the Seating Depth.

The Harmonic is shaped like an elongated Figure " 8 " because of all the Pressure, Torque and related Forces acting on the Barrel during a shot.

17 November 2003, 13:33

NotRicochet

I've wondered, too, where the exciting impulse(s) of the barrel vibrations come from? Chamber swelling under firing pressure? (If the barrel and receiver ring were perfectly symmetrical and free floated that couldn't whip the muzzle around, but with bedding on one side the barrel would be jacked up away from the bedding by firing expansion.) Changing the rate or extent of pressure rise at ignition could change the magnitude of the barrel vibration and conceivably the phasing of harmonics.

18 November 2003, 03:38

Dutch

Ricochet, one of the main sources of barrel vibration is because the barrel droops, and then the pressure straightens it out. Think of a garden hose, with and without the water turned on.

Secondly, the twist of the rifling torquest the barrel as it overcomes the inertia of the bullet (as it induces the spin). Thirdly, the bullet slamming into the rifling creates a vibration all it's own. Needless to say, the resulting vibration pattern is "somewhat complex". HTH, Dutch.

18 November 2003, 16:06

NotRicochet

Thanks, Dutch. I'd forgotten about the barrel droop, and hadn't thought about that straightening effect. I'd thought of the opposite effect, as the recoiling rifle with the center of pressure on the buttplate lower than the bore centerline rises with recoil, which would tend to make the barrel inertially bend down.

18 November 2003, 16:10

Gatehouse

quote:
Originally posted by Clark:
I have a friend who has a rifle that is most accurate with the bullet jammed into the lands.

He was hunting, and at lunch, he pulled the round out. But the bullet stayed in the lands and the case spilled powder all over in his rifle.

Not having a cleaning rod, he had to use someone else's rifle with the bullet and powder removed, shot muzzle to muzzle to get his bullet to pop out.

Now he carries a cleaning rod.

He took out the bullet and powder and SHOT the bullet out with primer power?

Interesting idea. I'll remember that one... [Smile]

19 November 2003, 06:08

<+P>

Tech Shooter on a other forum ( Snipers Hide )

Have a intresting idea on the theory of internal ballistics.

http://www.speakeasy.org/~cdlong/shock%20wave%20theory%20summary%20explanation.htm

Anders ï¿½sterberg SWEDEN

19 November 2003, 09:12

NotRicochet

That is a VERY interesting theory, and it makes a lot of sense. I'll be very interested to see how further testing works out.

19 November 2003, 16:27

Jameister

http://www.nookhill.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=6;t=007762

19 November 2003, 18:30

Paul5388

Jamiester,
I read your "smoke ring" thread and thought you might like this image.

It's a little bigger than what you described (6"/47 MK 16) and I have seen it make better rings, but you take what you can get in images!

20 November 2003, 03:02

Dutch

Anyone with the desire to "know" and the abilities to read mathematically oriented work should read this paper:

http://www.vni.com/successes/threerivers.html

Even without understanding the math, it helps understand what is going on during load development. HTH, Dutch.

20 November 2003, 05:53

FixedPower

Dutch, you were not kidding about the math! LOL. Those formulas were something else.

Thanks to all who responded to this topic. I am now a believer in the harmonic theory and seating depth relationship.

Maybe I am stubborn, but still reject the theory that the bullet may not hit the lands square and somehow be deformed and/or fly crooked. That just doesn't seem likely to me, at least under typical circumstances.

After all this theory, how about a practical question? When working up a new load, what seating depth increments do you try? 0.010"? 0.005"? What increment is too small to make a difference?

Thanks!

20 November 2003, 10:22

<rimshot>

quote:
Originally posted by Clark:
I have a friend who has a rifle that is most accurate with the bullet jammed into the lands.

He was hunting, and at lunch, he pulled the round out. But the bullet stayed in the lands and the case spilled powder all over in his rifle.

Not having a cleaning rod, he had to use someone else's rifle with the bullet and powder removed, shot muzzle to muzzle to get his bullet to pop out.

Now he carries a cleaning rod.

Now that's a creative solution! I never would have thought of it. I tend to forget just how much power is packed into one of those little primers.

rimshot

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