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Old August 28, 2011, 11:53 PM   #1
beex215
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how much does bullet depth matter?

i remember i changed my seating die and to get back to my original setting, i had to mess around seating some more bullets. the ones i did to re adjust i labeled as bad ammo. i went to the range and this bad ammo shot the best for some reason. i dont remember if it was seated deep or shallow. what are the effects of seating deep or not?
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Old August 29, 2011, 01:13 AM   #2
Jimro
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Seating deep increases pressure.

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Old August 29, 2011, 06:28 AM   #3
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Quote:
Seating deep increases pressure.
...exponentially, to add. In other words, it's critical to stay within reasonable OAL when experimenting with loads.
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Old August 29, 2011, 06:54 AM   #4
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as said... it can very quickly increase pressures...

also on some bullets / cartridges the amount of "jump" can greatly effect group size... I've seen the same load, with the bullet seated 3/16" out or in, cut group sizes in 1/2

I write down my best loads COL in my reciepe on the box ( I print everything on a piece of masking tape & put on each loaded box ) so if I had to clean my dies, that I know what to try to adjust them to
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Old August 29, 2011, 07:17 AM   #5
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I keep a binder with EVERY load I make. I list all components, powder charge, COL, velocity (if you have a chrono), and overall performance.

Keeping accurate records will make your life MUCH easier, and it only takes a second.

But to answer your initial ??, YES.
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Old August 29, 2011, 07:24 AM   #6
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I guess that depends on what you are loading. In a straight walled case reducing OAL will increase pressure, in a bottle necked rifle round not so much. In fact reducing OAL in a bottle necked rifle round usually decreases pressure not increases it.
What cartridge are we talking about here?
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Old August 29, 2011, 07:42 AM   #7
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I have found that in my search for accuracy in bottle-neck cartridges, that just touching the lands produces the best results. Others will disagree and get good results from other seating depth procedures.
I had a .257 Roberts Ruger M77 ("200th Year of Liberty") that had so much free-bore that I could not get near the lands with any bullet including the longest available for .257. I could never get better than inch and one-quarter groups out of that rifle. I have been able to get one minute of angle, or better with all the modern rifles I have owned except that one. All the others, I could seat the bullets to just touch the lands. I do not say that the reason I could not get M.O.A. out of that rifle is because of the seating depth; there might have been some other reason. So, take it for what it is worth.
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Old August 29, 2011, 07:51 AM   #8
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I think the Weatherby line of cartridges usually uses a longer throat... maybe to insure the huge cartridges don't go over pressure ( as pressures are normally greater if the bullet touches the lands ) & yet the Weatherbys often shoot MOA or better... so I don't think it's cut & dried that one way works better than the other... though if I remember correctly, I think cast bullets usually shoot better up against the lands ( or was it back a ways ) ... I think it's best against, & probably that the bullet deforms less ???
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Old August 29, 2011, 10:59 AM   #9
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As stated, seating depth affects pressure and accuracy. To bad that you didn't write down your load information since the load appeared accurate.

It's best to record load information for every round loaded. Minimum amount of information to write down would be primer type, case used, powder, powder weight, cartridge length, bullet make, type and weight.
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Old August 29, 2011, 11:26 AM   #10
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As with everything else, length matters. It effects pressures and accuracy, but so does bullet length, ogive, bullet weight, it all matters. That is why there is loading data published so you have a range to start and you can adjust form there if need be until you get the accuracy you want or your gun/rifle blows up.
Oh yea, loading records are important too.
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Old August 29, 2011, 04:54 PM   #11
beex215
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i loaded 9mm and 308. yes, i obviously didnt write it down and i will next time.
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Old August 29, 2011, 05:13 PM   #12
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BEEX... it's really easy ( & a great idea ) to just keep a roll of 1" - 1.5" masking tape at your bench, & write all the load reciepe on the tape... a trim length is nice ( in case your shooting a semi auto or other headspace on the case mouth cartridge ) as well as a COL... if nothing else, so you can re-setup your dies between cleanings, if you had a stuck case, or in the instance that you load 2 cartridges ( like 38 special & 357 magnum ) out of the same dies... I like to put dates of loading at the bottom, just so I know I'm getting a rotation on my ammo
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Old August 29, 2011, 08:53 PM   #13
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9mm is highly sensitive to seating depth.
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Old August 29, 2011, 08:59 PM   #14
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Thanks Magnum, I have been using post it notes in my ammo boxes. Didn't think about masking tape. I like that guess I will buy a roll tomorrow. The things you learn here...

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Old August 30, 2011, 06:20 AM   #15
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You can also use your computer to make labels. I do my reloads in batches so I give each box a box number and then I write down how many times that box has been reloaded. What powder how many grains, what bullet and how many grains, what cases and primers I used. Case trim length and Ogive length, date reloaded, and what rifle/pistol/revolver they are for.
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Old August 30, 2011, 09:18 AM   #16
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Beex215,

I wish there was a simple answer to your question, but there isn't. Different chambers and bullets like different seating. At one time, most benchrest shooters used Dahermit's method of kissing the rifling, though most use some number close-to but not quite touching, now. Even they often don't actually know what's best. The Precision Shooting Reloading Guide has the example of a benchrest shooter accidentally turning his seating die micrometer the wrong way when he changed bullets, ending up with them 0.050" off the lands instead of his intended 0.020", and it cut his groups roughly in half. Berger used to recommend their VLD's be loaded to touch the lands, but got so many reports of individuals doing better with them off the lands that they explored this and found their bullets actually did best anywhere from very close to the lands to as far back as 0.165" off the lands (see Berger's letter in the first post here). It just depends on the chamber and bullet. Item 3. near the top of this old page is a nice anecdotal example of tuning seating depth producing good results even in a shot out throat.

As to pressure, it does change with bullet seating depth, but not unilaterally. Below is a pressure vs. seating depth curve taken from data in the old U. of Michigan study. What you see is the bullet touching the throat at the left and seating deeper as you go the right. Pressure drops as you do that because the bullet has to jump further to the lands, allowing more gas to bypass it before it obturates the bore. As it goes deeper still, pressure starts to rise again because the deeper seating reduces the amount of space the powder has to burn in, and that finally starts raising pressure enough to overwhelm the gas bypass pressure drop effect.



When pressure changes, barrel time (how long the bullet takes to get out of the gun once the powder starts building pressure) changes, too. If the barrel time was just right to exit the bore at a sweet spot in the phase of the barrel deflection from recoil (often called "vibration" even though the string isn't actually plucked until the bullet clears the muzzle), that will be moved off the sweet spot if you change the pressure enough. That makes it hard to tell if the seating depth or pressure is being tuned when you move the bullet. The only way I know to separate the two is to use a very reduced load when tuning seating depth so barrel vibration doesn't come into it as much. Bullets start to move at about the same start pressure regardless of the charge (within reason - not when you get below about 12,000 psi) so the lighter load doesn't seem to affect the best bullet position much. If you are unsure, retest the best depth with two different powder charges and see if it continues to be the best spot.

Once you have a seating depth established with modest loads that don't rattle the gun a lot, then it's time to tune the powder charge. You'll generally find you need to do it separately for each primer you try. I like Dan Newberry's round robin variant on ladder shooting for this purpose, as it can be used at 100 yards more readily than the old style ladder. You can read about his whole method, here.
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Old September 2, 2011, 04:01 AM   #17
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As usual UncleNick has provided great info and not much to add.

I usually will start off with my OAL set to fit my magazine and feed well. Some of the newer plastic tipped bullet will hang the front gap between the mag and receiver in a couple of my rifles, if seated out to just fit so sometimes they are a bit shorter to make sure this doesn't happen.

Once I have a reliable lenght, I will work up to the max listed load looking for pressure and checking velocity. If I manage to get there, I usually back off about a grain or so, and then start reducing my OAL looking for a sweet spot. If I find something I might keep the load if not I might back off the powder just a tad more and repeat.

Using this method I have yet to flatten out a primer or even have a jump in velocity, usually it is the other way around just as Nick has shown. I can usually hit something in the first .075" or of depth change, which really has little effect on my pressure or velocity with the medium to slower powders I tend to use.
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Old September 2, 2011, 09:22 AM   #18
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Beex215,

It is REALLY important to specify what cartridge you are loading when you ask a question. Even your answer that you load 9mm and .308 Winchester really doesn't help us give you the correct information, because those two roounds are VERY different with respect to over-all-length / seating depth relationship to pressure.

As others have stated, 9mm is very sensitive to seating depth. A quote from Speer manual # 10 says "... loads that produced 28,000 CUP went to 62,000 CUP when bullets were purposely seated 0.030" deeper!" (page 349). The effect is due to the very small powder space remaining under the bullet in this small case (which causes a small change in seating depth to make a big change in the ratio of powder space for the two seatng depths), coupled with the very non-linear burn rate of smokeless powders as a function of pressure while burning.

Bottleneck rifle cartridges like the .308 have a much larger powder space, so the ratio of powder spaces remaining for seating depth changes like 0.030" are not really significant to pressure. What is significant in the .308 is how far the bullet moves before it encounters the rifling and is inhibited in its acceleration to some extent. When the bullet is not accelerating as rapidly as before, it allows the pressure to build more rapidly, which causes the powder to burn more rapidly, which builds pressure more rapidly, etc. until the pressure is high enough to accelerate the bullet fast enough to stop the pressure buildup. It is a very dynamic process that is hard to model and dangerous to assume follows simple non-dynamic relationships like the "Perfect Gas Law." Unclenick's graph is a good EXAMPLE of the effect this has on pressure, but different guns, powders, bullets, etc will give somewhat different relationships of pressure with seating depths for the same cartridge. And different cartridges will have even more variation in the relationship.

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Old September 2, 2011, 09:53 AM   #19
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I'd like to emphasize SL-1's last point. The graph is of a round nose bullet. RSI has a pressure plot for a 70 grain 6 mm Speer TNT SP bullet in a 6PPC for which pressure dropped 20% just going from touching the lands to being a mere 0.030" off the lands (first plot under "Other Sample Traces" scrolling 2/3 down this page). So nose shape and chamber shape will make the slopes of the curve change, sometimes dramatically.
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Old September 2, 2011, 05:02 PM   #20
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Kind of a thought experiment here:

I'm wondering if the initial slope of the lands would have anything to do with how well it takes a bullet that is touching it or is set a bit off the lands.

For example, if the lands have a moderate slope, a bullet might do very well being slightly set back. But if the lands have very little slope, the bullet might do better if it's just slightly touching the lands.

Or the reverse could be true, but it might be worth checking.

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Old September 2, 2011, 05:27 PM   #21
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I use these Avery peel off label on my ammo boxes and their free software to print them with OAL, powder, bullet, etc. They stay on and are easy to remove:

http://www.avery.com/avery/en_us/Pro...bels_05440.htm
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Old September 2, 2011, 05:30 PM   #22
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It means everything to us,, we ordered all the modified cases from Hornady of the rifles we have just to make dang sure we don't leave any avenue out.....
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Old September 2, 2011, 08:58 PM   #23
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Bullet depth/cartridge length is a complicated subject. The easiest answer is "yes" but the reasons, as you see above, vary widely. For a modern smokeless rifle most folks feel the "jump" from case to lands is critical so they try to determine the optimal cartridge length or "jump" for their rifle. For BP cartridge rifle shooters bullet depth and cartridge length are critical because the powder charge must be compressed and the "jump" distance can be a factor as well. Can be a delicate balancing act. Auto pistols that headspace on the case mouth can be picky about bullet seating depth as well, feeding issues can result from long or short cases in addition to the overpressure issues mentioned above. Even revolvers can be a bit picky but that often has more to do with case volume and tension issues.
The above is all generalities, not an absolutely reliable predictor for your firearm, just a starting place. Point is; if you have a load that works very well for you, measure (and record!) everything you can about it so you can duplicate it. If you have a load that shows promise then bullet depth is a variable that should be considered.
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Old September 3, 2011, 09:02 AM   #24
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Here is a good article from Barnes. Scroll down to "from the lab".

http://www.barnesbullets.com/resourc...rnes-bullet-n/
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Old September 3, 2011, 10:00 AM   #25
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It's an interesting experiment but curiously uninformed on a couple of points. For example, they seem surprised that velocity doesn't change proportionally with peak pressure. Peak pressure is never proportional to velocity because it's determined by how much gas is made in what degree of expanded space there is behind the moving bullet. As peak pressure goes up, burning rate increases so the bullet has less time to move forward. That means progressively higher peaks occur in ever decreasing volume which makes the pressure growth exponential.

Velocity, on the other hand, is determined by average pressure in a bore, not peak pressure. The average pressure allows the gas to fill the whole barrel by the time the bullet exits, which is a more constant volume than the changing expanded volume extant at the peak. The two are not independent of one another, but it's not uncommon for peak pressure to change three times as much as the average pressure, percentage-wise.

For another thing, they are varying pressure and velocity with a fixed powder charge. This changes barrel time. The higher the peak, the larger the portion of bullet acceleration that occurs early in its travel, so it goes a little faster down the rest of the tube, shortening barrel time. This moves the bullet in and out of sweet spots in the barrel deflection and vibration. It is therefore no surprise to find they can tune to more than one sweet spot by varying seating depths with with a fixed charge.

I thought it was pretty widely known that you can often find a seating depth sweet spot near the lands and another with the bullet bearing surface about one caliber into the case mouth. Obviously some chamber and bullet combinations don't really allow a choice between the two, but it's not uncommonly observed in those that do, much to the relief of those of us who have magazine fed match rifles.
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Last edited by Unclenick; September 3, 2011 at 02:22 PM. Reason: typo fix
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