I'm developing new loads for my new F/TR gun. It's a 308 Win, Bartlein 30" 1 in 10 twist heavy varmint contour, throat depth .085. I'm loading the Berger 185 grn match hybrid in Lapua brass, CCI BR2 primers. I'll be trying Varget, VHT N540 and VHT N550 powders initially. Based on my understanding of the powder burn cycle here is a multi-part question. If pressure spikes at the moment the bullet starts to move and then quickly begins to drop, wouldn't pressure spike slightly a second time if the bullet is seated a fair distance off the lands since the bullet movement is impeded at this point? Also, how would this affect the resulting bullet flight and stabilization? Is the best precision load better served by seating at or extremely close to the lands?
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Re: Bullet seating depth and powder pressure
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If pressure spikes at the moment the bullet starts to move and then quickly begins to drop, wouldn't pressure spike slightly a second time if the bullet is seated a fair distance off the lands since the bullet movement is impeded at this point?</div></div>Pressure is not likely to spike as the bullet starts to move. The inital movement of the bullet will be from the primer blast; correct? If seating into the lands, this is changed.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If pressure spikes at the moment the bullet starts to move and then quickly begins to drop, wouldn't pressure spike slightly a second time if the bullet is seated a fair distance off the lands since the bullet movement is impeded at this point?</div></div>Pressure is not likely to spike as the bullet starts to move. The inital movement of the bullet will be from the primer blast; correct? If seating into the lands, this is changed.
Re: Bullet seating depth and powder pressure
As I understand, a bullet that is not seated into the lands, that has to jump to the rifling, allows for a slower buildup of the initial pressure spike. A bullet jammed into the rifling won't move until the pressure spike is far higher.
This is supposed to explain the longer leade in rifles that are chambered in 5.56mm or the long leade in the M-24 in .308.
As I understand, a bullet that is not seated into the lands, that has to jump to the rifling, allows for a slower buildup of the initial pressure spike. A bullet jammed into the rifling won't move until the pressure spike is far higher.
This is supposed to explain the longer leade in rifles that are chambered in 5.56mm or the long leade in the M-24 in .308.
Re: Bullet seating depth and powder pressure
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MtnCreek</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If pressure spikes at the moment the bullet starts to move and then quickly begins to drop, wouldn't pressure spike slightly a second time if the bullet is seated a fair distance off the lands since the bullet movement is impeded at this point?</div></div>Pressure is not likely to spike as the bullet starts to move. <span style="color: #FF0000"> The inital movement of the bullet will be from the primer blast; correct?</span> If seating into the lands, this is changed. </div></div>
This seems to be a commonly held belief, but my experience has been that it's incorrect. In my last batch of ammo I had 3 rounds that I missed putting powder into. When fired all I got was a click. I thought they may have been bad primers.
When I took the 3 rounds home and started pulling the bullets I found the bullets had blackened bases, and that I had just forgotten to put powder in the case. The last one was the only one I measured the base to ogive length of, but the bullet had not moved, not even a little. This was in a 308 AR with 0.002" neck tension and no crimp. Bullets were seated 0.140" off the lands.
The primers I used were CCI BR-2's which is quite mild. Results may vary based on which primer one were actually using, case volume, and neck tension as well as whether or not the volume the powder takes up in the case has any effect before it ignites, but based on my anecdote I assume the projectile doesn't move until the powder is burning.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MtnCreek</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If pressure spikes at the moment the bullet starts to move and then quickly begins to drop, wouldn't pressure spike slightly a second time if the bullet is seated a fair distance off the lands since the bullet movement is impeded at this point?</div></div>Pressure is not likely to spike as the bullet starts to move. <span style="color: #FF0000"> The inital movement of the bullet will be from the primer blast; correct?</span> If seating into the lands, this is changed. </div></div>
This seems to be a commonly held belief, but my experience has been that it's incorrect. In my last batch of ammo I had 3 rounds that I missed putting powder into. When fired all I got was a click. I thought they may have been bad primers.
When I took the 3 rounds home and started pulling the bullets I found the bullets had blackened bases, and that I had just forgotten to put powder in the case. The last one was the only one I measured the base to ogive length of, but the bullet had not moved, not even a little. This was in a 308 AR with 0.002" neck tension and no crimp. Bullets were seated 0.140" off the lands.
The primers I used were CCI BR-2's which is quite mild. Results may vary based on which primer one were actually using, case volume, and neck tension as well as whether or not the volume the powder takes up in the case has any effect before it ignites, but based on my anecdote I assume the projectile doesn't move until the powder is burning.
Re: Bullet seating depth and powder pressure
pressure does not spike as soon as the bullet moves, but typically when the bullet is a ~1/3 the way down the barrel.
i dont think bullet jump changes the time to max pressure by allot , it might change the total pressure , but i imagine the spike will still occur at the same time/place.
its possible to have a secondary spike , or two spikes , but the second spike would be at a significantly lower pressure then the first. I think that would have more to do with the powder then the seating depth
typically seating bullets closer the lands produces better groups. You would have to do a seating depth test
pressure does not spike as soon as the bullet moves, but typically when the bullet is a ~1/3 the way down the barrel.
i dont think bullet jump changes the time to max pressure by allot , it might change the total pressure , but i imagine the spike will still occur at the same time/place.
its possible to have a secondary spike , or two spikes , but the second spike would be at a significantly lower pressure then the first. I think that would have more to do with the powder then the seating depth
typically seating bullets closer the lands produces better groups. You would have to do a seating depth test
Re: Bullet seating depth and powder pressure
<span style="font-style: italic">If pressure spikes at the moment the bullet starts to move and then quickly begins to drop, </span>
As mentioned above, that's not true for rifles. Chamber pressure starts with a very small jump at the moment of primer detonation but peak pressure rises as powder ingnition begins. At some point the pressure is sufficent to start the bullet moving and as soon as it does the burn space volume rapidly enlarges and pressure is moderated by that increase. Peak chamber pressure usually occurs something like 2 1/2 to 3 1/2 thousanth of a second, the bullets are typically only about 3 inches down the rifling at that point. The bullet continues to accellerate, expanding the burn chamber and dropping pressure as it does while the powder continues to burn and add more velocity.
".<span style="font-style: italic">..wouldn't pressure spike slightly a second time if the bullet is seated a fair distance off the lands since the bullet movement is impeded at this point?"</span>
No. First, you need to understand the time/pressure curve IS a 'spike' by definition; all you read about (horrors!) 'spiking' pressures is just the pressure going higher than expected. All the somber stuff you read about pressure 'spikes' is misleading nonsense trying to sound knowledgeable.
Second, a bullet seated at, or close to, the lands has to wait for the preassure to get high enough to jam it into the lands; that jam pressure can be pretty high. On the other hand, a bullet with a bit of jump will have attained sufficent velocity and inertia to carry itself into the lands much easier than from a standing start. Meaning the pressure from an equal powder charge will be less if the bullet gets a running start.
<span style="font-style: italic">"Also, how would this affect the resulting bullet flight and stabilization?"</span>
It doesn't. The only thing affecting flight is the bullet's BC and the stabilazation is controlled by the velocity and barrel twist.
<span style="font-style: italic">"Is the best precision load better served by seating at or extremely close to the lands?"</span>
No. Seating at or slightly into the lands is a BR techinque that works well with their small cartridges and tight chambers using very low bullet tension so touching the lands has little harmful effect and instead helps get primary powder burn high enough for consistancy of the peak pressure.
A few sporting rifle owners with standard chambers have claimed to shoot better at the lands but most of us do better for both accuracy and velocity with bullet jumps from maybe 20 thou to as much as 5 times that much.
<span style="font-style: italic">If pressure spikes at the moment the bullet starts to move and then quickly begins to drop, </span>
As mentioned above, that's not true for rifles. Chamber pressure starts with a very small jump at the moment of primer detonation but peak pressure rises as powder ingnition begins. At some point the pressure is sufficent to start the bullet moving and as soon as it does the burn space volume rapidly enlarges and pressure is moderated by that increase. Peak chamber pressure usually occurs something like 2 1/2 to 3 1/2 thousanth of a second, the bullets are typically only about 3 inches down the rifling at that point. The bullet continues to accellerate, expanding the burn chamber and dropping pressure as it does while the powder continues to burn and add more velocity.
".<span style="font-style: italic">..wouldn't pressure spike slightly a second time if the bullet is seated a fair distance off the lands since the bullet movement is impeded at this point?"</span>
No. First, you need to understand the time/pressure curve IS a 'spike' by definition; all you read about (horrors!) 'spiking' pressures is just the pressure going higher than expected. All the somber stuff you read about pressure 'spikes' is misleading nonsense trying to sound knowledgeable.
Second, a bullet seated at, or close to, the lands has to wait for the preassure to get high enough to jam it into the lands; that jam pressure can be pretty high. On the other hand, a bullet with a bit of jump will have attained sufficent velocity and inertia to carry itself into the lands much easier than from a standing start. Meaning the pressure from an equal powder charge will be less if the bullet gets a running start.
<span style="font-style: italic">"Also, how would this affect the resulting bullet flight and stabilization?"</span>
It doesn't. The only thing affecting flight is the bullet's BC and the stabilazation is controlled by the velocity and barrel twist.
<span style="font-style: italic">"Is the best precision load better served by seating at or extremely close to the lands?"</span>
No. Seating at or slightly into the lands is a BR techinque that works well with their small cartridges and tight chambers using very low bullet tension so touching the lands has little harmful effect and instead helps get primary powder burn high enough for consistancy of the peak pressure.
A few sporting rifle owners with standard chambers have claimed to shoot better at the lands but most of us do better for both accuracy and velocity with bullet jumps from maybe 20 thou to as much as 5 times that much.
Re: Bullet seating depth and powder pressure
I usually end up with the bullet slightly stuck into the lands when I forget to put the powder in. Sometimes really stuck and comes out of the case when it is ejected, sometimes it ejects with the case.
I usually have the bullet stuck in the throat with the .223 but I've always attributed it to the smaller lighter bullet being easier to move. With the increased volume of the .308 case, if there isn't powder in it, it has a lot of room for the gasses to expand and trying to push a heavier bullet, well the heavier it is, the harder it is to get it started moving?
Rambling thoughts reading all of this . . . .
I usually end up with the bullet slightly stuck into the lands when I forget to put the powder in. Sometimes really stuck and comes out of the case when it is ejected, sometimes it ejects with the case.
I usually have the bullet stuck in the throat with the .223 but I've always attributed it to the smaller lighter bullet being easier to move. With the increased volume of the .308 case, if there isn't powder in it, it has a lot of room for the gasses to expand and trying to push a heavier bullet, well the heavier it is, the harder it is to get it started moving?
Rambling thoughts reading all of this . . . .
Re: Bullet seating depth and powder pressure
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fuzzball</div><div class="ubbcode-body">"...usually end up with the bullet slightly stuck into the lands when I forget to put the powder in."
Forget to put the powder in? Goodness. </div></div>
Ok, so I'm not perfect but I learn fast. That hasn't happened in a while. The little light on my press now allows me to visually check every round now. Checking the powder level is part of my process now.
Ok, who has never loaded a round without powder?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fuzzball</div><div class="ubbcode-body">"...usually end up with the bullet slightly stuck into the lands when I forget to put the powder in."
Forget to put the powder in? Goodness. </div></div>
Ok, so I'm not perfect but I learn fast. That hasn't happened in a while. The little light on my press now allows me to visually check every round now. Checking the powder level is part of my process now.
Ok, who has never loaded a round without powder?
Re: Bullet seating depth and powder pressure
<span style="font-style: italic">"Ok, who has never loaded a round without powder?"</span>
Well, I haven't. Nor ever done a squib or double charge either. Yet. But there's still time, I've only been reloading for rifles, handguns and shotguns since '65. But I'm an old slow learner that has always been attentive to serious details.
Good luck!
<span style="font-style: italic">"Ok, who has never loaded a round without powder?"</span>
Well, I haven't. Nor ever done a squib or double charge either. Yet. But there's still time, I've only been reloading for rifles, handguns and shotguns since '65. But I'm an old slow learner that has always been attentive to serious details.
Good luck!
Re: Bullet seating depth and powder pressure
Interesting info here. I guess from what is being said that with a 338LM, one would probably not want to load to the lands?
Please verify.
Thanks
Interesting info here. I guess from what is being said that with a 338LM, one would probably not want to load to the lands?
Please verify.
Thanks
Re: Bullet seating depth and powder pressure
"...with a 338LM, one would probably not want to load to the lands?"
Well, that's true. But, a 338 LM is just another bottleneck sporting cartridge, it's not anything special or unique for reloading. Seating at the lands requires more chamber pressure, and time, for the bullet to move but if we reduce the charge properly we can compensate for that delay and subsiquent pressure rise in any cartridge.
The supposed value of seating into the lands is greatly misunderstood and it means less to accuracy than many seem to suspect. SOME B.R. shooters have seated that way for decades but it's only for the small cartridges they shoot and the very light bullet tension they usually have. IF you want to load like them you'll need to have a rifle made and chambered like theirs.
A 338 mag of any kind doesn't qualify as a BR rig so you'll probably do much better with a good bit of bullet jump and 'normal' bullet grip.
"...with a 338LM, one would probably not want to load to the lands?"
Well, that's true. But, a 338 LM is just another bottleneck sporting cartridge, it's not anything special or unique for reloading. Seating at the lands requires more chamber pressure, and time, for the bullet to move but if we reduce the charge properly we can compensate for that delay and subsiquent pressure rise in any cartridge.
The supposed value of seating into the lands is greatly misunderstood and it means less to accuracy than many seem to suspect. SOME B.R. shooters have seated that way for decades but it's only for the small cartridges they shoot and the very light bullet tension they usually have. IF you want to load like them you'll need to have a rifle made and chambered like theirs.
A 338 mag of any kind doesn't qualify as a BR rig so you'll probably do much better with a good bit of bullet jump and 'normal' bullet grip.
