pressure
- Thread starter sako2
- Start date
It depends on the bullet design and how much empty space (including the space between the powder grains) there is in the case behind it. Here's a plot showing the difference between touching the lands and being 0.030" off the lands in a 6 PPC.
That plot shows a 20% increase in pressure contacting the lands. However, the bullet has a fairly typical ogive radius coming off the shoulder at the forward end of the bearing surface like you would see on Spitzer-nose bullets.
The plot below is from data in Dr. Lloyd Brownell's seminal work on pressure at the University of Michigan under a grant from DuPont in the mid-'60s. In this case, it was a round nose bullet. Round nose bullets have an ogive that slopes off the same shoulder much more gradually until it gets to the round tip.
In that situation, notice the peak pressure at contact is only 10% above the low point.
What is going on is the further you seat the bullet, the bigger the gap is between the ogive and the surface of the throat of your chamber. It's called an annular ring gap because it is donut-shaped. It is the space through which bypass gases escape after the case neck has released the bullet, but before the bullet has accelerated enough to close it. The bypassing gas makes it hard for pressure to rise and uses up some gas. Also, the more running start the bullet has, the faster the jump into the throat is completed and expansion begins, limiting the peak pressure value.
The round nose bullet has such a gradual taper to its ogive that it has to be pushed a lot deeper into the case to open that gap an equal amount. Therefore, it has to have a bigger jump (be seated deeper) to make the same change in the annular opening and change the bypass gas flow. In the case of that plot, it needs about 0.25 inches to find a pressure low point.
As far as pressure rising while you seat the bullet deeper, it increases exponentially as the bullet goes deeper in. This is the reason the second curve has a dip and then rises again as the bullet goes deeper. So, near to making throat contact, the proximity of the ogive to the throat changed pressure fastest with each increment of seating depth, while far enough in, the change in pressure due to the bullet taking up powder space becomes the faster-changing influence. That dip at the bottom is where the two rates of change are equal.
That plot shows a 20% increase in pressure contacting the lands. However, the bullet has a fairly typical ogive radius coming off the shoulder at the forward end of the bearing surface like you would see on Spitzer-nose bullets.
The plot below is from data in Dr. Lloyd Brownell's seminal work on pressure at the University of Michigan under a grant from DuPont in the mid-'60s. In this case, it was a round nose bullet. Round nose bullets have an ogive that slopes off the same shoulder much more gradually until it gets to the round tip.
In that situation, notice the peak pressure at contact is only 10% above the low point.
What is going on is the further you seat the bullet, the bigger the gap is between the ogive and the surface of the throat of your chamber. It's called an annular ring gap because it is donut-shaped. It is the space through which bypass gases escape after the case neck has released the bullet, but before the bullet has accelerated enough to close it. The bypassing gas makes it hard for pressure to rise and uses up some gas. Also, the more running start the bullet has, the faster the jump into the throat is completed and expansion begins, limiting the peak pressure value.
The round nose bullet has such a gradual taper to its ogive that it has to be pushed a lot deeper into the case to open that gap an equal amount. Therefore, it has to have a bigger jump (be seated deeper) to make the same change in the annular opening and change the bypass gas flow. In the case of that plot, it needs about 0.25 inches to find a pressure low point.
As far as pressure rising while you seat the bullet deeper, it increases exponentially as the bullet goes deeper in. This is the reason the second curve has a dip and then rises again as the bullet goes deeper. So, near to making throat contact, the proximity of the ogive to the throat changed pressure fastest with each increment of seating depth, while far enough in, the change in pressure due to the bullet taking up powder space becomes the faster-changing influence. That dip at the bottom is where the two rates of change are equal.
Aguila Blanca
Staff
When are you going to write a book?
Only if you are forced to seat it such that the boattail takes up more space in the case than the flatbase does. But if the bullets weigh the same and have the same nose profile and are seated to the same COL, then no. The boattail in that circumstance will have less bearing surface in the case neck, so the net volume taken up in the neck will be the same.
If you use published load data that produced 60,000 psi pressure and 3,000 fps in the barrel it was developed in, don't assume it'll be the same in your barrel. It could be higher or lower due to powder lot, bullet pull force, barrel internal dimensions and how the rifle's held when fired.
Its interesting to see that on the Chrono Graph (I bought the LabRadar and working on a report)
I find the COAL at the Ogrive for a given bullet, seat some below that and then use a small press at the range to adjust the others and see if accuracy wise there is a difference (that is subjective as I told the guy at the range why I did not used a Shilen vs a Lilja barrel, I am not worth wasting a a Lilja on !)
Also interesting is identical bullets and the Ogive variance can be really large on random ones.
And to see a cartridge fired with a wilder FPS and hit smack in the group and one with a spot on to the average go off, hmmmm
I used to think 1/4 MOA was realistic with my load setups. Not sure how many levels up you have to take it, but 1/2 MOA now is impressive (for me) when you factor in all the possible variations.
One I am pondering is do you do an individual bullet Ogive check so you can throw out the odd balls? Something like Berger might be more consistent but cost so much more it is non viable for me (and shoot more or spend more and able to shoot less even if you can get them?)
What professional bench rest shooters pull off gets into the I would not believe it if 100s if not thousands of people see it.
jetinteriorguy
New member
I tried an experiment with seating depth once. I backed off the seating stem on every load and slowly increased the depth to a predetermined depth so all twenty rounds were seated identical according to my Hornady comparator. It made no discernible difference in five shot groups compared to just setting the seating stem and then just seating them like I’ve always done. So at least at my skill level as a shooter all these small random differences in bullet shape etc. don’t seem to matter. I’m on my good days about a 3/4 MOA shooter and on average a 1 MOA to 1 1/4 MOA shooter.
Is there any noted difference on rounds with a concave open end v a boat tail?
Having pulled .45 rounds out of a backstop with partial jackets - i.e. open backs - make me wonder this.
One obvious factor would be more space in the case due to the projectile shape, but wondering if the concave may actually flare out upon hitting the barrel and giving you a better seal down the length of the barrel.
Having pulled .45 rounds out of a backstop with partial jackets - i.e. open backs - make me wonder this.
One obvious factor would be more space in the case due to the projectile shape, but wondering if the concave may actually flare out upon hitting the barrel and giving you a better seal down the length of the barrel.
Shooting .223 reloads through a Savage Axis. Sometimes a round would not feed right and the bullet would get pushed into the case. Once the bullet got buried so deep it looked doubtful my Cam-lock bullet puller would have anything to grab hold of... so I foolishly fired the round. Result was 1fps difference from the rest of the 49 reloads and same POI.
My guess... if you jam a bullet you will get pressure spikes that need to be compensated for, but a bullet far from the lands will allow so much pressure to escape that it's not a big deal, or I was stupid lucky. Using Benchmark powder with 53-55gr bullets.
My guess... if you jam a bullet you will get pressure spikes that need to be compensated for, but a bullet far from the lands will allow so much pressure to escape that it's not a big deal, or I was stupid lucky. Using Benchmark powder with 53-55gr bullets.
Best accuracy happens when the bullet diameter is at least a few ten-thousandths inch larger than the barrel's groove diameter. That doesn't change the bullet's shape at its back end or tail.
It's easier to make hollow point bullet's heel more dimensionally uniform than full metal jacket ones with a hollow or open base. Gas escapes more evenly around the heel as it exits the muzzle.
Sierra Bullets learned this in the late 1950's noting some lots of their 30 caliber 165 grain BTHP hunting bullets shot more accurate than their 30 caliber FMJBT match bullets. They experimented with the design that led to their 308 caliber 168 grain HPBT International match bullet. Their 180 and 200 grain FMJBT match bullets were soon changed to hollow point boattail ones.
jetinteriorguy
New member
If this happens on a regular basis you might want to try crimping your bullets, or at least fix what’s causing this to happen.
That is not how I would put it.
First, unless you get into one of those odd areas of too little powder created a pressure pulse, then carbon on a case is too little pressure to seal it.
I get leary of gas escape aspects and its a gun crud up as well as brass clean up, I move them up.
I have never seen carbon when I got the pressure too high.
As you correctly noted, hard bolt lift, sticky extraction and there are no ejector wipe marks are saying you are fine. You can assess if you go higher based on what your load level is vs the books and how close you are.
Primers are low on my list, its not that I don't look at them but its a primer compared to a lower load of the same powder and the other factors that are possibly relevant.
Simply put, if I have any of the bolt stuff I stop and go down. A primer alone, no. Too many causes to be a "here be dragons"
I look at it as going into a mine field. Ok, someone marked it (developed loads and came up with a max) and I am going to be carefull.
But, if you are all good and your load is doing what you want (accuracy be it bench rest or hunting good) then you are in calm waters.
My take is I go with what my barrels tell me. If I could get under 1/2 MOA and I was shooting a ridiculously low 1500 fps with the 06 (however I got there) it would be, cool, I am good with this.
I marvel at the people who want 2800 FPS out of an 06 (175 gr area) for what? Hard on you, hard on the barrel and the drop at the further ranges is not changed to any real degree (at 600 yards its still going to be a lot so a 1/2 inch better is?)