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  #1  
Old 03-21-2010, 07:59 PM
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Bullet seating depth


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There's plenty of discussion about bullet seating depth (overall length), but not about the actual depth of the bullet in the case. When I seat 40 gr bullets close to the lands with my 221 Fireball, the bullets are being seated into the case just a little more than 1/16" (.075"). For this reason I used flat-base bullets. They're tight enough I can't pull them out by hand, but I wonder if they wouldn't be easily misaligned. I can't recall ever reading anything about minimum seating in the case. Just curious if anyone has any knowledge or comments on this.
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Old 03-21-2010, 08:48 PM
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Welcome to the forum. Rules are to join in, have fun, and play nicely with the rest of us kids.

You've got your terminology overlapping a little. COL (cartridge overall length) is what is usually given. It is the top to bottom length of a finished cartridge. Seating depth is the right term for the actual depth of the bottom of the bullet below the mouth of the case. It used to be given in some of the old load manuals. My 1972 Hornady, for example, gives seating depths rather than COL's. I think the manual owners saw liability issues with asking people to figure out the COL that would give them the listed seating depth, so they switched to providing COL's; at least in some instances.

True seating depth is what you need to get matching pressure, assuming the bullet is pretty far from the lands (pressure also goes up as it gets close to the lands). For that reason, I think it is a good idea to record seating depths for your loads for ease of swapping bullets around. The basic calculations are:

Seating Depth = Bullet Length + Case Length - COL

If you have a desired seating depth and want to know what COL will give it to you, just rearrange the terms as follows:

COL = Bullet Length + Case Length - Seating Depth

Hope that helps.
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Last edited by unclenick; 03-21-2010 at 08:51 PM.
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Old 03-21-2010, 08:58 PM
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That sounds pretty shallow - 1/16". I can;t imagine seating my bullets that shallow, but it appears it has worked for you.

There is a general rule, since you asked. I believe it is to seat the bullet to a depth equal to the diameter of the bullet. So......224" or 1/4" for the 221 Fireball. I could be wrong about that and, even if I'm off a little, I don;t think it's a requirement, but rather a rule.
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  #4  
Old 03-21-2010, 09:14 PM
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Yes, that's exactly the way I came up with the .075" for the depth of the bullet in the case. Case length = 1.394, bullet length = 0.562", and COAL = 1.956". So the base of the bullet is seating .075" into the case. That puts the bullet .007" from the lands, using the Stoney Point gauge. My concern is whether it's a bad idea to seat the bullet that shallow into the case. I would think that it would be pretty easy to twist the bullet sideways and pull it right out of the case if I wanted to, or at the very least it could end up not being concentric (I have a CaseMaster gauge so I could play with it and find out.) Is there a rule of thumb for minimum depth of seating, like maybe half of the neck length? I've been reloading since 1975 and I can't remember ever reading such a thing.
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Old 03-21-2010, 09:29 PM
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If the loading manuals just added this:

If the loading manuals just added bullet length information, seating depth would be easy to establish. Of course that information would allow handloaders to readily use the data with bullets from other manufacturers.
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Old 03-21-2010, 09:31 PM
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"Is there a rule of thumb for minimum depth of seating, like maybe half of the neck length? "

Rule of thumb: diameter of bullet.
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  #7  
Old 03-21-2010, 09:33 PM
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Originally Posted by lantyr View Post
Yes, that's exactly the way I came up with the .075" for the depth of the bullet in the case. Case length = 1.394, bullet length = 0.562", and COAL = 1.956". So the base of the bullet is seating .075" into the case. That puts the bullet .007" from the lands, using the Stoney Point gauge. My concern is whether it's a bad idea to seat the bullet that shallow into the case. I would think that it would be pretty easy to twist the bullet sideways and pull it right out of the case if I wanted to, or at the very least it could end up not being concentric (I have a CaseMaster gauge so I could play with it and find out.) Is there a rule of thumb for minimum depth of seating, like maybe half of the neck length? I've been reloading since 1975 and I can't remember ever reading such a thing.
I have found that 40gr boattail bullets seated out to the same OAL in my .223 as 50 grainers have more bullet run out than 50gr bullets. They aren't in the case very far at all , but there is enough neck tension that they can't be pulled by hand. If I try to correct the run out with my Hornady bullet run out tool it's very easy to lose neck tension. I've decided it's better to seat them a little deeper and get rid of excess run out. Now I have to do accuracy tests to test my theory. It's a dirty job but somebody's gotta do it !
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  #8  
Old 03-21-2010, 09:36 PM
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The old rule of thumb was - seat to a minimum of one bullet diamenter. Seat .224" for a .224, .308" for a .308, etc., etc.

That get blown into a cocked hat when wanting to seat as close to the lands as possible, or wanting to seat to just clear the magazine well. Then, there are folks that say .010" t0 .020" off the lands, Barnes bullets says to allow at least .050" for their solid core bullets and then those that say jocky the depth to find the most accurate performing seating depth. Other folks like to seat to the cannalure ring, if present. Whew!!

My method is to first make up a dummy case and find where the maximum length is to touch the lands. Then, I find out where the maximum functioning length of the magazine will be. Usually, there's a great difference with these two measurements. A bullet seated one diameter deep will afford good neck tension. I let the magazine length control the COAL. This always give me good depth and neck tension.

If you're loading for a single-shot, of prefer to load singly, the above doesn't matter - seat out as far as desired, but try not to touch the lands.
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  #9  
Old 03-21-2010, 09:59 PM
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Thanks for all the thoughts. Not that it matters, but the neck of the little Fireball is only about 0.192" long, measured on the outside of the case, so seating deeper than that is not an issue anyway. With the bullet seated .075" into the case, it is seated a little more than 1/3 the neck length into the neck. I guess I'll experiment more with the longer bullets like 50 gr, and also with shorter OAL (bullet much farther from the lands.) In the end, whatever works, works. (My loads fit the magazine of my CZ rifle OK. Inadequate neck tension could be a problem - - I've had some cases show a considerable amount of smoking, and I sometimes get a lot of smoke from the muzzle - - maybe incomplete powder burning - - I'm going to chronograph my loads this week, which I haven't done yet on this rifle.) This is a new rifle and I'm just beginning to work up a load for it, and I sure don't have any experience with the little Fireball, so this is an interesting challenge....

Last edited by lantyr; 03-21-2010 at 10:03 PM.
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  #10  
Old 03-22-2010, 04:56 AM
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Give this article a read and you might find you're worrying a whole lot about the wrong thing. In my experience, seating less than one caliber is just asking for concentricity problems, which contributes more to accuracy than the distance a bullet travels before it's in the lands.

http://www.24hourcampfire.com/reloading_subpage.htm
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Old 03-22-2010, 06:21 AM
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I have some loads that are very, very accurate that are barely seated in the case. Maybe a tenth of an inch. This in the 6.5x55 Swede which has a long throat. So yeah it can be done.

They do work fine but care has to be taken in the field to not handle roughly. With new brass it has not been a problem. On the other hand I have not tested extensively to see whether there would be a loss of accuracy if they were deeper. Maybe when this box is gone.....

My take on the issue used to be to always seat as long as possible for accuracy. While this worked for the guns I was using, I also took great pains to ensure that bullets were seated concentric to the case, and that the cases were sized to fit the chamber well. After reading about the OCW method of load development, I am going to go that path in the future. See the "sticky" in the Ballistics topic. The seating depth will be treated as a secondary variable; when the powder charge is settled on, then I will start tweaking the depth.

Those Swede reloads with the bullet sticking waaaaay out of the case sure look neat though!
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  #12  
Old 03-22-2010, 03:02 PM
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Originally Posted by MikeG View Post
I have some loads that are very, very accurate that are barely seated in the case. Maybe a tenth of an inch. This in the 6.5x55 Swede which has a long throat. So yeah it can be done.

Those Swede reloads with the bullet sticking waaaaay out of the case sure look neat though!
In my daughters 6.5x55, I can set the 140gr A-Max on top of a sized case, slowly chamber it, and then turn it muzzle down and hear the bullet go "clink" when it hits the lands.

I just seat them to fit the magazine and they shoot wonderfully.

For the OP, I usually try to keep loads that will function form the magazine. In the singles I keep at least a caliber length in the case just so I feel good about it.
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Old 03-22-2010, 03:19 PM
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When all else fails, just hold a bullet up next to the neck so the the base of the bullet and the bottom of the neck align - and seat to that depth. Except for very heavy or very light bullets, that usually works out just fine.

Many older rounds were designed that way: they adjusted the neck length so that the most common bullet came to the base of the neck, and the mouth came at the cannelure (if any). Check a .30-30 with a 170-gr bullet sometime. Made for each other.
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  #14  
Old 03-24-2010, 01:43 AM
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There is a conundrum with all this talk of COL compared to seating depth, one goes in hand with the other.......... to a point!

The main reason that there has been a rule of thumb of using one calibre as the minimum seating depth is due to the fact that the neck of any case acts as a guide for the initial travel of the bullet, which is supposed to align it with the leade of the chamber, helping concentricity from shot to shot.

I dabbled with this theory in my 25-06 and 75gr Sierra HP's.
I could not get the bullets within .080" of the rifling, even with them seated as shallow as .020" into the case, but I did discover pretty quickly that this shallow seating wasn't very good for accuracy. It would shoot shotgun patterns with this seating depth!
If I seated them .257" into the case it would consistently shoot a ragged hole, even though the bullets were 'miles' from the lands.

The only reason I could come up with for the bad accuracy is that not enough pressure, and therefore not enough velocity, was being produced to get the bullet to have a short interval from case mouth to chamber, which in turn caused the bullets to tip and engage the rifling at an angle which was detrimental to any consistency from shot to shot.
Also, chronographing those loads would show a velocity difference I have never seen before, anywhere up to 200fps difference, whether higher or lower than the loads that were seated at one calibre. The loads at one calibre seating depth were very consistent with a swing of only 25 fps +/-!
In fact, after several sessions with the chrono I was getting a consistent average velocity reading of 3706 fps, this occurred on 8 different occasions, the other 4 were only slightly different with an average velocity reading of 3699fps and 3702fps! All tests were taken with a 40 round string!

I almost shot out my barrel doing this test, just to see how important seating depth actually was!
I believe it's very important!

I also took this info with me when developing loads for my 300WinMag, with it's short .265"neck, seating depth turned out to be almost more important in a consistent load than did an optimum powder charge............... I did say almost!
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Old 03-24-2010, 12:24 PM
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If there is smoke on your case I was told that means not enough peak pressure. If your using published data for powder amounts. then that answers your own question. Not seated deep enough = not enough peak pressure. Good rule of thumb is 1 case diameter of seating depth. Like others have said, neck is only thing aligning bullet to bore so adiquate seating depth is important. If shallow seating does happen to work for someone,,, then great for them keep on a truckin...
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Old 03-24-2010, 01:53 PM
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MM,

What you describe makes perfect sense. Part of the reason is start pressure. You may have noticed a fired case's neck is normally expanded to the chamber neck diameter except right at the edge of the mouth, which remains curled in? That is because, as pressure builds, the case expands to fill the chamber diameter, but the neck has to expand from where the bottom of the bullet bearing surface is squeezed by neck tension, then forward from there to let go of the bullet. As that forward rolling neck expansion arrives at the case mouth, it starts to bleed gas out, so the pressure differential between the case and the outside is lost, and is thus unable to expand the lip of the mouth like the rest of the neck.

What's important about that is, during the forward rolling expansion of the neck the bullet largely stays put. That allows pressure to build. At its release, the bullet has gas blowing past it via the edge of the case mouth and into the bore, but because the pressure has already grown, the bullet pops forward into the bore pretty fast. It's like a champagne cork that way. During the short time there is gas bypassing the bullet, the pressure rate of rise in the chamber is slowed, but once the bullet moves forward enough to obturate the bore, it continues building well.

If the bullet is shallowly seated, pressure won't have built nearly as much by the time the bullet has been released. This means the bullet moves toward the throat more slowly while allowing gas bypass. If the distance it has to go is short, the gas bypass period is still pretty short, and pressure may then go on to build normally. But if the distance the bullet has to travel to obturate the bore is long, gas bypass from a shallowly seated bullet will go on for a relatively long time, which greatly reduces peak pressure. A little difference in neck tension, a little shift in the powder position at ignition, or anything else that might slightly change the exact pressure at which the bullet is released and at which gas bypass commences will then have a greatly exaggerated influence on final peak pressure. This is due to varying the initial burning rate of the powder by varying the pressure via the amount of gas that bleeds off. Powder burning rate varies greatly with pressure, but especially greatly at pressures barely adequate to keep the powder burning. In other words, it's like taking all the factors that cause normal variation in velocity and magnifying their effect on peak pressure several fold.

Below is a graph taken from data in the 1965 U of Michigan study of cartridge pressure. It is for a round nose bullet in a .30-06 case. The round nose bullet is almost 0.9" long and the short ogive means it doesn't have to stick out very far to touch the lands. As a result, even when the bullet is touching the lands in the Springfield '03 barrel used in the tests, it is still pretty well seated into the neck. As it is seated still deeper, gas bypass that did not occur when the bullet touched the lands starts. and becomes increasingly greater. This drops starting pressure and thereby lowers the resulting peak pressure because the powder is burning more slowly when the bullet is getting into the bore. Once the bullet is seated deeply enough, the decrease in peak pressure due to gas bypass starts to be offset by the decrease in case volume the bullet has under it. Once that volume gets small enough, start pressure is assisted in its rise by commencing to burn in the smaller volume, and that raises pressure again.

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Old 03-24-2010, 08:55 PM
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I did some shooting over the chrongraph today and the results are pretty eye-opening, with some very consistent velocities, and some very erratic velocities. I'll take a look at the data tomorrow and see what it means. Gotta head for bed now....
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Old 03-24-2010, 09:50 PM
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MM,

What you describe makes perfect sense. Part of the reason is start pressure. You may have noticed a fired case's neck is normally expanded to the chamber neck diameter except right at the edge of the mouth, which remains curled in? That is because, as pressure builds, the case expands to fill the chamber diameter, but the neck has to expand from where the bottom of the bullet bearing surface is squeezed by neck tension, then forward from there to let go of the bullet. As that forward rolling neck expansion arrives at the case mouth, it starts to bleed gas out, so the pressure differential between the case and the outside is lost, and is thus unable to expand the lip of the mouth like the rest of the neck.

What's important about that is, during the forward rolling expansion of the neck the bullet largely stays put. That allows pressure to build. At its release, the bullet has gas blowing past it via the edge of the case mouth and into the bore, but because the pressure has already grown, the bullet pops forward into the bore pretty fast. It's like a champagne cork that way. During the short time there is gas bypassing the bullet, the pressure rate of rise in the chamber is slowed, but once the bullet moves forward enough to obturate the bore, it continues building well.

If the bullet is shallowly seated, pressure won't have built nearly as much by the time the bullet has been released. This means the bullet moves toward the throat more slowly while allowing gas bypass. If the distance it has to go is short, the gas bypass period is still pretty short, and pressure may then go on to build normally. But if the distance the bullet has to travel to obturate the bore is long, gas bypass from a shallowly seated bullet will go on for a relatively long time, which greatly reduces peak pressure. A little difference in neck tension, a little shift in the powder position at ignition, or anything else that might slightly change the exact pressure at which the bullet is released and at which gas bypass commences will then have a greatly exaggerated influence on final peak pressure. This is due to varying the initial burning rate of the powder by varying the pressure via the amount of gas that bleeds off. Powder burning rate varies greatly with pressure, but especially greatly at pressures barely adequate to keep the powder burning. In other words, it's like taking all the factors that cause normal variation in velocity and magnifying their effect on peak pressure several fold.

Below is a graph taken from data in the 1965 U of Michigan study of cartridge pressure. It is for a round nose bullet in a .30-06 case. The round nose bullet is almost 0.9" long and the short ogive means it doesn't have to stick out very far to touch the lands. As a result, even when the bullet is touching the lands in the Springfield '03 barrel used in the tests, it is still pretty well seated into the neck. As it is seated still deeper, gas bypass that did not occur when the bullet touched the lands starts. and becomes increasingly greater. This drops starting pressure and thereby lowers the resulting peak pressure because the powder is burning more slowly when the bullet is getting into the bore. Once the bullet is seated deeply enough, the decrease in peak pressure due to gas bypass starts to be offset by the decrease in case volume the bullet has under it. Once that volume gets small enough, start pressure is assisted in its rise by commencing to burn in the smaller volume, and that raises pressure again.

At least we are on the same page!
Thanks for that chart, I haven't seen or heard of it before, it definitely corresponds with my theory!
I was very confused by the above outcome for quite some time, believing that the closer the bullet was to the lands would improve pressure and velocity to a point, but the exact opposite occurred!
More surprising was how inaccurate the outcome was, no other change was made to the load other than decreasing seating depth. I did not have a pressure measuring device when I conducted the above test, all I had to go on was my chrony.

In reference to my 300WinMag, I have found that start pressure is very important to load development, if the start pressure is low, pressure stays that way for the entire travel of the bullet and gives poor erratic velocities, but if it's high, velocities are very uniform. Seating depth and neck tension are the biggest contributing factors in uniformly high start pressure.
Without the Pressure Trace II system I would never have known any of this, I would have just assumed that a particular bullet/powder combo wasn't suitable and dismissed them from further testing, but I found that the seating depth and neck tension could make all the difference!
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Old 03-25-2010, 05:01 AM
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Nick,

Would the presence of a firm crimp tend to mitigate the effects shown in the above graph? There was a very recent thread wherein the OP found that crimping his loads made them more accurate and now I'm wondering if this particular phenomenon might explain why?
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Old 03-25-2010, 06:28 AM
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I was kind of wondering the same thing when I read this, and got to thinking that crimping might be just what the doctor ordered for my .223, which seems to be having problems with vertical stringing...certainly, the pressure should be much more consistent throughout ignition?
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