Seating depth tendencies

R

robinny

New member
I've done some searches and haven't really found a good answer to this question. Maybe I'm just not a good searcher or haven't searched hard enough.

If I find that a particular bullet shoots best out of my gun when seated further off the lands than recommended, is it likely that most bullets will shoot better with more jump than recommended? Not that every bullet is going to shoot best when seated the same distance from the lands, but will they all trend in one direction relative to the norm?

Here's an example. I tested some 85 gr HPFB jacketed bullets with 42 grains of IMR4350 out of my .243, and found that as I increased seating depth from 0.010" of jump to 0.050" my groups steadily decreased from 2.5 MOA to 0.6 MOA. So compared to the common recommendation I've read to start around 0.020" off the lands, it seems this gun likes this bullet to be seated a bit deep.

I'd like to try loading the Barnes 80 gr TTSX in the same gun. Slightly different weight, different bullet construction. Barnes recommends starting at 0.050" off the lands and increasing seating depth in 0.025" increments. Given the jacketed HPFB performance, would it make sense to start at something more like 0.075" or even 0.100" and avoid wasting components and time, or is it possible that this bullet might work better seated shallower than recommended?

I guess the boiled down version of the question is: do specific guns tend to "like" all bullets seated shallow or deep, or will they shoot some bullets better shallower and some deeper?

Thanks for any insight you might have.
 
I'm honestly not sure. Never thought about it. My proecss has always been
work up to max looking for pressure.
back the load off and test for seating depth with the berger method.
play with powder from max down a bit to see if groups tighten up.
Write the load down on a 3x5 card and stash it away.

I have always done this for new bullets... would be nice to skip the test, but I suspect it is needed...
 
I don't know officially, but my experience is it depends on the bullet at least in the same gun.

Some like close and some like jump and has been noted, quite a bit of jump (Weatherby magnums are noted mandatory for it due to his nutty pressures)

I just start loads at .015 off and then play with it.
 
It may simply be your bullet likes the increased velocity it is getting by being seated deeper. I would try seating it normally and slightly increasing your powder charge if you are not near a Max load.

Don
 
Thanks, fellas.

You make an excellent point, USSR. I'm not near max, so I'll try another few rounds at 0.020" or thereabouts and see what a couple increments of increased charge reveal.
 
Robinny,

Welcome to the forum. Please drop by the General forum to post in the Introduce Yourself thread at the top. Also, please check out the forum rules if you haven't already.

In a rifle cartridge, as you have, the normal behavior is for pressure and velocity to be high when you jam the lands—about 20% higher than at SAAMI seating depth is common. This raises the velocity as well. As you seat deeper, pressure and velocity drop, but only to a point. Below that point, the pressure and velocity start to go up again.

What is happening has to do with gas bypass. In general, pressure in the case peels the neck off the bullet from the rear rolling forward before the bullet moves a significant distance. With the neck open, gas can leak out along the sides of the bullet and go around the nose and into the bore as long as the bullet isn't yet in the throat and sealing the leak path (obturating the bore). This keeps pressure from rising. The further back from the throat the bullet is, the greater the leak, which then gradually closes off as the bullet starts moving to the throat.

Realizing it only takes a few tens of pounds force to seat a jacketed rifle bullet in a case, it greatly surprises people that the bullet isn't moving sooner and faster. The reason is the time frame is so short that the bullet's inertia keeps it from advancing more quickly. On a flat surface you can put your car in neutral and push it a foot forward easily, but you can't push hard enough to get it up to 100 mph by the time you hands have moved it that far. That would require a great deal more force, and that is what is going on with the bullet. H.P. White Laboratories did a lot of basic firearms measurements and found they could not detect rifle bullet movement until pressures were at about 10,000 psi for normal high power rifle loads. DuPont measured some not moving until about 20,000 psi. Anyone who has ever stuck a bullet in a throat with the primer in an uncharged case will know that kind of pressure is not needed to move the bullet, but primer unseating has a lot more time to occur than firing a bullet does. The inflections in the pressure rise curve that may be observed when the bullet jumps to the throat are on the order of 50 microseconds into the powder burn, with some variation, and that much pressure is needed to move the bullet from case-to-throat that fast. (See pages numbered 16 through 18 in http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3866/bac6873.0001.001.pdf?sequence=5.)

The bottom line is you are adjusting pressures and velocities and barrel times when you change seating depth.
 
2 pics are worth 1000 words

same rifle, same bullet weight, same powder, even the same charge weight
 

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Holy Target Spread, hounddawg!

robinny,

Just for clarification, did you stay at 42gr on your powder charge when playing with the OAL?
 
Houndawg,

I'm curious, what was the overall loaded length of the various rounds, compared to the standard Max overall loaded length?

Were they longer or shorter than SAAMI spec??
 
Shane Tuttle said:
Holy Target Spread, hounddawg!

robinny,

Just for clarification, did you stay at 42gr on your powder charge when playing with the OAL?
Click to expand...
Affirmative. Trying a method I read wherein one tests seating depth increments all at the same load, at or near listed starting load for the bullet in question. After finding a seating depth node, testing then moves to increasing powder charges until pressure signs become evident or an accuracy node is found.
 
That assumes an accuracy node will stay put as powder charge changes. Normally, though, increased charge shortens barrel time and takes you off the node if the difference is high enough to do so. So you may end up going back and forth for a bit. With luck, you'll be narrowing the variables with each try.
 
hounddawg said:
2 pics are worth 1000 words

same rifle, same bullet weight, same powder, even the same charge weight
Click to expand...
Thanks, hounddawg. Those images do indeed imply a great deal relative to my question.

Although a very simple "no" would also have said just about the same thing. ;)
Seriously, thanks for taking the time to post them. They are quite illustrative.
 
Unclenick said:
That assumes an accuracy node will stay put as powder charge changes. Normally, though, increased charge shortens barrel time and takes you off the node if the difference is high enough to do so. So you may end up going back and forth for a bit. With luck, you'll be narrowing the variables with each try.
Click to expand...
Yes, I suppose since both variables affect accuracy, you could wind up tweaking back and forth a few times. Everyone seems to swear by their method, and for someone relatively new at this it can be confusing. Of course anyone who paid attention in high school science class knows to only change one variable at a time, and as you point out eventually you can narrow both down.

In the case of my example, those were inexpensive pulls that I intend to use for practicing shooting technique, so I don't really need to get max velocity or a lot more accuracy out of them. But since they were cheap I don't mind burning up a lot to get some load development practice, too. Only problem is the somewhat uncontrollable variable of bullet inconsistency from being loaded and pulled. I've been sorting them by visible quality as well as weight, so trying to reduce the effect of that variable.
 
Houndawg,

I'm curious, what was the overall loaded length of the various rounds, compared to the standard Max overall loaded length?

Were they longer or shorter than SAAMI spec??
Click to expand...

Just as a guess I would say longer. The Berger may have been magazine capable but I do not recall now. Both loads were intended for single feed in a bolt rifle. I do all my measurements base to ogive unless I am planning on a magazine feed so I have no clue what the exact COL is on any loads that are not designed for a AR. It was a "learning" barrel and while it is still on a action after 3 K of hot rounds down the tube it can barely hold 3/4 MOA at 100 now and is on the schedule for replacement. I could go down and get some current measurements bu with throat erosion they would not be anywhere near what was shot at those targets

Thanks, hounddawg. Those images do indeed imply a great deal relative to my question.
Click to expand...

no problem, I figure that visuals provide more impact. Those are my reloading log now. I keep jpg's of all my targets and review them
 
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RC20 said:
Some like close and some like jump and has been noted, quite a bit of jump (Weatherby magnums are noted mandatory for it due to his nutty pressures).
Click to expand...
Several cartridges have the same SAAMI spec 65,000 psi max average pressure as the 300 Wby Mag; 270 Winchester, for example. What's nutty about that?
 
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Great discussion.

Unclenick said: " As you seat deeper, pressure and velocity drop, but only to a point. Below that point, the pressure and velocity start to go up again."

Is that because there is a critical point where the internal volume of the case is reduced by the increase in seating depth?
 
I’ve having an age-related brain nerve dysfunction with this discussion from the Berger link:

“Effects of Seating Depth / COAL on Pressure and Velocity

The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well-known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.”

OK, a bullet deeply seated by SAAMI specifications results in less internal space, higher pressure and velocity.

If you seat that bullet longer, there is more internal space, but you get a reduction in pressure and velocity.

So far, so good.

Now you add more powder to the longer seated bullet and you get greater velocity at the same pressure as the deeper -seated bullet. This is where the brain-nerve dysfunction enters. Is the author really saying the pressure is equal to the deeper seated bullet and thus the velocity is higher than the previous longer seated bullet, but might well be equal to the deeper seated velocity?

I can’t wrap my head around the same pressure producing a greater velocity.
 
I find Bryan Litz's remark that the scratch ring around the bullet is where the ogive starts curving down from the bullet's largest diameter not possible. All the bullets I've measured diameters at that point are a few thousandths less than the bullet's largest diameter.

The ogive has to start at the bullet's largest diameter. The scratch ring moves further forward from that point as throat angle increases.

His "Chamber throat geometry" diagram uses terminology that may confuse some. What he calls "throat" is actually "freebore." The throat starts at freebore diameter then angles down to bore diameter.
 
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