Thread: Pressure / barrel speeds?

Originally Posted by Nick-D

Heavier pills = bigger pills = more surface area touching the rifleing = more friction = more pressure

Has more to do with F=ma

They are some good ejector marks!

Pressure is when you kick open the bolt, the primer falls out of the oval pocket and part of your bolt is gone.


image by AckleyImproved, on Flickr


image by AckleyImproved, on Flickr

Originally Posted by Tahr-Bound

Pressure is when you kick open the bolt, the primer falls out of the oval pocket and part of your bolt is gone.


image by AckleyImproved, on Flickr

Fark!!!!!

I fixed Willis bolt after some hot loads, out tahr shooting he smashed it up with a rock off come handle, she was all locked up hard, couldn't get bolt up had to undo the barrel to back pressure off to open bolt

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280Ack Imp using a load I had worked up to pressure and backed off to a safe level and fired many times. It was pissing down at the time and my only guess is water got into the chamber and caused a huge pressure spike. Scared me a lot.

Thats not from my reloading knowledge (dont have any of that). This is just an observation based on my understanding of physics. (Civil engineer once upon a time).
@Gibo

Originally Posted by Nick-D

Thats not from my reloading knowledge (dont have any of that). This is just an observation based on my understanding of physics. (Civil engineer once upon a time).
@Gibo

All good

Originally Posted by Banana

Has more to do with F=ma

Actually, f=ma has little do do with it, other than determining 'a' as the force does not change(charge weight stays the same). The spike in pressure is cause by the increased time the projectile stays in the barrel. This increases the peak pressure as more powder is converted to gas (ie more pressure) before the projectile moves enough for the peak pressure to drop away. Much the same as a pressurised water system.

The Big determining factors on this increase in barrel time are going to be mass or force (pill size & charge), static friction (friction build up before the prjectile moves) and of course barrel friction (givin that the projectile is slighly over bore). This is why they say do not use reloading data from 1 120 grain projectile to the next, as the frictional forces have a significant effect on initial pressure, veloctiy, and peak pressure achieved in the chamber and barrel. (as does projectile weight and charge size)

Again, no expert on ballistics, just observations based on what I remember from my engineering days

Maybe I should have written it as a=F/m. Same force but more mass = less acceleration. Same powder burn rate (roughly) and less projectile acceleration, so pressure peaks in less projectile travel.

Originally Posted by BRADS

Ok when your brass does this
This wasn't my loading....it came with a gun I brought.....
Guy said they where all sweet, he seemed two no his stuff,
the first few where all fine
I really spat the dummy bolt lift was REALLY hard. I was glad it was in a Sako, a lesser action the results may have been a very different......
Lesson.....never use loads your given
Let's use this as a worse case example

That definitely is a good one in the pressure department, I bought a Remington 788 in 243 a few years ago it came with some reloads from a well known ammo re-loader the shop told me who made the ammo all good with me but he never said fire at your own risk. First and second shot was sweet third shot big orange flash in my face with a big bang, bullet tumbles down range and smacks side on into target. Look at rifle smoke coming out of places it should not be coming out of along with the odor of burnt oil, go to open bolt hello it is jammed solid took it to feller who made ammo, the feller hammers bolt open, extracts case with case firmly implanted into bolt, hammers case out of bolt face primer drops on ground with a huge tit where it extruded into firing pin hole, primer pocket almost doubled in size and all case head stamping was just a bright shiny surface and all stampings gone, needless to say rifle was going to take a fair bit to fix.

Originally Posted by Banana

Maybe I should have written it as a=F/m. Same force but more mass = less acceleration. Same powder burn rate (roughly) and less projectile acceleration, so pressure peaks in less projectile travel.

Indeed, The burn rate will change with the pressure spike. Cant remember which way though haha. Same principles as an engine.

lets take two bullets of the same type but different weights out of the same rifle.
say 120gr 6.5BT and 140gr 6.5BT
Correct me if I'm wrong but, say 48gr of a 2209 moves the 120gr at 2900.
But as this is well over max it would be dangerous for the 140gr
Is this because the lighter bullet has gone further down the barrel with the same powder charge so the pressure will be less than a heavier bullet that has travelled less distance therefore leaving less empty volume behind the bullet for pressure increase.
Or is that a load of bollocks and it all happens in the chamber in fraction of seconds before the bullet even gets a few mm down the barrel.
Or Something else.

Originally Posted by jakewire

lets take two bullets of the same type but different weights out of the same rifle.
say 120gr 6.5BT and 140gr 6.5BT
Correct me if I'm wrong but, say 48gr of a 2209 moves the 120gr at 2900.
But as this is well over max it would be dangerous for the 140gr
Is this because the lighter bullet has gone further down the barrel with the same powder charge so the pressure will be less than a heavier bullet that has travelled less distance therefore leaving less empty volume behind the bullet for pressure increase.
Or is that a load of bollocks and it all happens in the chamber in fraction of seconds before the bullet even gets a few mm down the barrel.
Or Something else.

Fantastic question i will put mine on option 1

Its a compounding effect.
the more pressure that builds the faster it burns and the more pressure builds.
Light some powder poured on some concrete... pretty unimpressive.
Things that increase pressure but not limited to are.
increased weight
increased neck tension
hard jackets
less jump
increased bearing surface
etc etc etc.

this is why we work up and drop back a bit and work up again if changing anything dramaticly

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