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Seating depth, powder charge and speeds

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bstomper

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I am hoping someone can clear this up for me.
without getting to technical, is it the amount of pressure the powder charge gives in a certain case volume, that causes the bullet speed. Basically, seating the bullet longer will give more room for powder in the case. Will this added powder give the bullet more speed. Or is the added powder simply just filling the case to give the same pressures and speeds I had before
For ease of explanation I will use random numbers. If I have a load at a COAL of 3.300"and a powder charge of 55 grains with a given pressure and a speed of 2900 fps, if I seat the bullet longer to a COAL of 3.400" and still 55gns, obviously the case capacity is bigger now so I am assuming the pressure should be lower. If I bump up the power charge to give me the same pressures I had before, will the speed still be the same at 2900 fps or will it be faster.
My rifle can only handle X amount of pressure so In my mind it is the pressure inside a given space that gives speed and not the amount of powder, the powder only fills the case to give the pressure. Hope this makes sense. lol
 
Adding more powder will increase the velocity for same seating depth. If you reduce seating depth a good bit, you may also increase velocity. Seating too long, may also increase velocity if too close/ in the lands.

What loading long typically allows you to do, is load more of the slower burning powder that would tyically cause you to have compressed loads.

When doing seating tests, I have not seen velocity changes with the fine tuning depths. I have seen it increase with significant seating depth changes (deeper).

Make sense??? Not really...It's all voodoo man
 
Generally speaking, from SAAMI spec, loading longer increases case volume and decreases pressure, which allows more powder which increases velocity (as previously stated, usually using slow burning powders).
At either extreme, either seating deeper or longer, will increase pressure either by decreasing case volume or by bullet getting too close to the lands.

For example, with the same powder charge in my 300 RUM, starting .015 off and increasing seating depth in .015 increments, I would see an approximate 10 fps decrease each step as I got further from the lands til about .045 off.
 
This is an interesting topic. Tidbits to add to the soup.....where extreme velocities are most often achieved are the intersection of decreasing seating depth to increase powder capacity to 'accelerate' the bullet for a longer amount of time. This is really only consistently true with slow burning powders. The powder is not ignited all at once. The charge begins to ignite, reaches a pressure level that initiates movement of the bullet, then a long push of the powder continuing to ignite over the length of the barrel. This is why shorter barrels typically prodice less velocity. Even though what we hear and feel are a quick snappy 'bang', what is really going on is an extreme 'wooooosh' the legth of the 'wooosh' is determined by the powders quantity and burn rate. Think of the ignition more like a bottle rocket and less like a destructive explosive.

So, the amount of neck tension and even crimping have sigificant affects on the pressure curve, as does the temp/intensity of the primer. It is very scientific, but not easily calculated which formula will yield best velocity, but that is the fun of reloading.
As stated previously, longer seating, the most qty of the slowest powder that will ignite and expand the most in your barrel with your bullet will usually yield the best velocity.
 
The longest COAL that chambers in your rifle and is .005 off the lands has the greatest speed potential if you can fill it with slow enough burning powder - i.e. slightly compressed at near max pressure. If you just partially fill it with faster buring powder you may be going slower than if you seated the bullet deeper.
 
bstomper said:
I am hoping someone can clear this up for me.
without getting to technical,
Click to expand...
You want a bumper sticker answer, to a technical question?!!?

The bumper sticker sticker answer to your question is;
The pressure under the curve over time, is what gives the bullet velocity.

For a little more detail, without going very technical:
Dr. Brownell pressure tested this long long ago, people ignored it because they only wanted cliff notes answers. One of those cliff Notees, was Hartmut. Because he clearly ignored pressure tested data, and made an egregious error in QL assuming linear pressure response. The next Cliff Notee was a prominent bullet company who wasn't doing any pressure testing, and released a whole series of manuals predicated upon this incorrect bottom of bullet seating depth and pressure... I think you can see how this cycle of stupidity has snowballed.

Case volume and geometry both, affect a powders burning rate. But it isn't linearly and simply, unfortunately. Within what most anyone would consider reasonable seating depth ranges for a bullet, the pressure differential will be minor enough to be inconsequential; Which is why Ackley was always wrong.

Now when you get to the extreme ends of seating depth range, then pressure *can* change quickly. Pay attention to the word in asterisks.
With a legacy powder as you get close to the lands, you lose bleed -off, and if obturation and main charge information line up, you'll get drastically higher pressure than you think you should. *Can* was important, because is you have a progressive powder in this situation, then they can often times not have this pressure spike as they adjust their burning rate.

Alternatively when you seat deeper you have more bleed off and lower pressure, relatively speaking. However there is a point where pressure suddenly spikes and begins exponentially increasing as you seat deeper.
 
Bang4theBuck said:
Even though what we hear and feel are a quick snappy 'bang', what is really going on is an extreme 'wooooosh' the legth of the 'wooosh' is determined by the powders quantity and burn rate. Think of the ignition more like a bottle rocket and less like a destructive explosive.

So, the amount of neck tension and even crimping have sigificant affects on the pressure curve...
Click to expand...
This is the correct answer. SAAMI specifies a peak pressure for a given chamber, say 65K psi. It is not just the peak pressure that affects velocity, but the entire area under the pressure/time curve. In other words, a very sharp pressure/time curve at 65K psi will produce less velocity than one that reaches 65K psi and holds the pressure for a while before dropping. The key is matching the powder burn rate/properties that produce the largest area under the pressure/time curve without exceeding the maximum peak pressure for the chamber.
 
Last edited:
Darkker said:
You want a bumper sticker answer, to a technical question?!!?

The bumper sticker sticker answer to your question is;
The pressure under the curve over time, is what gives the bullet velocity.

For a little more detail, without going very technical:
Dr. Brownell pressure tested this long long ago, people ignored it because they only wanted cliff notes answers. One of those cliff Notees, was Hartmut. Because he clearly ignored pressure tested data, and made an egregious error in QL assuming linear pressure response. The next Cliff Notee was a prominent bullet company who wasn't doing any pressure testing, and released a whole series of manuals predicated upon this incorrect bottom of bullet seating depth and pressure... I think you can see how this cycle of stupidity has snowballed.

Case volume and geometry both, affect a powders burning rate. But it isn't linearly and simply, unfortunately. Within what most anyone would consider reasonable seating depth ranges for a bullet, the pressure differential will be minor enough to be inconsequential; Which is why Ackley was always wrong.

Now when you get to the extreme ends of seating depth range, then pressure *can* change quickly. Pay attention to the word in asterisks.
With a legacy powder as you get close to the lands, you lose bleed -off, and if obturation and main charge information line up, you'll get drastically higher pressure than you think you should. *Can* was important, because is you have a progressive powder in this situation, then they can often times not have this pressure spike as they adjust their burning rate.

Alternatively when you seat deeper you have more bleed off and lower pressure, relatively speaking. However there is a point where pressure suddenly spikes and begins exponentially increasing as you seat deeper.
Click to expand...
This ^^^
 
VenatusDominus said:
. In other words, a very sharp pressure curve at 65K psi will produce less velocity than one that reaches 65K psi and holds the pressure for a while before dropping.
Click to expand...

This^^^

When I showed a guy the traces in one of my Creedmoor's, where I'm running 15,000psi lower peak pressure, but only lost 20fps. Thought he was going to try and restart the Salem Witch trials🤣
 
Tiny Tim said:
Generally speaking, from SAAMI spec, loading longer increases case volume and decreases pressure, which allows more powder which increases velocity (as previously stated, usually using slow burning powders).
At either extreme, either seating deeper or longer, will increase pressure either by decreasing case volume or by bullet getting too close to the lands.

For example, with the same powder charge in my 300 RUM, starting .015 off and increasing seating depth in .015 increments, I would see an approximate 10 fps decrease each step as I got further from the lands til about .045 off.
Click to expand...
It was always my understanding, the RUM gets a lot of it's extra speed from the exta long throat in a Sammi chamber. The projectile has more time start moving before contacting the lands. The closer, the slower.
 
azarcher10x said:
It was always my understanding, the RUM gets a lot of it's extra speed from the exta long throat in a Sammi chamber. The projectile has more time start moving before contacting the lands. The closer, the slower.
Click to expand...
Not really, you could load longer and go faster . Just like wbee if you single shot.
 
I also think a part of the missing equation is the weight of the projectile and the fit in the barrel. It take more energy to start a 220 grain bullet than a 55 grain bullet. Thus your peak pressure changes. With a tight fitting slug in the barrel Or a heavier projectile you will also have a sustained "peak", and lower speeds.
 

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