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Chamber pressures

 
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  #8  
Old 01-18-2013, 07:45 PM
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Re: Chamber pressures

Conventional "wisdom" aside, seating deeper with rifles has been known to produce lower peak pressures since at least the 1930s when it was documented to do so in arsenal tests of that period, at least until the seating is about 1/4" below the common military seating depth for .30-06. And, as Hornady has illustrated since at least 1980, we can count on it to some degee in virtually anything.

In fact, so far as I have been able to learn, seating deeper has little effect on peak pressure in any cartridges except the very few tiny internal volume-high pressure-fast powder-minimum leade pistol rounds like the 9 and .40/10 mm autoloader cases when using moderately heavy to heavy for caliber bullets.
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  #9  
Old 01-18-2013, 08:43 PM
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Re: Chamber pressures

Quote:
Originally Posted by Mikecr View Post
If you're talking reasonable seating adjustments, deeper seating(by itself) increases pressure a relatively small amount due to reducing case capacity and increasing load density. Reaching the lands causes a bigger step change upward in pressure(+~8Kpsi) even after the smaller reduction of pressure with shallow seating to get there.
None of this is an issue as you would back down a bit with testing after any significant change.

But if you seat so deep that bullet bearing runs into neck-shoulder junction, and if you have a donut developing, pressures can again step change big upward. This would show up as bad performance first, followed by classic pressure signs.
My comment was about your first paragraph, which does not appear to be correct and is somewhat misleading regarding pressure/seating depth. I just wanted to show that others found the opposite.
Now, perhaps you have done your own pressure trace tests, I don't know, but if you are just repeating "Conventional Wisdom" then that is more like an ol' wive's tale, and needs to be challenged so we can all learn.

regarding "Donuts" I do believe you on that, but only so far as there could be no room for release of the bullet, as the brass could be jammed in the neck clamping down on the bullet.

regarding chamber pressures, I think of it this way, the bullet has little to do with pressure until it encounters the rifling. A bullet is only a fraction of a pound, yet chamber pressures are measured in Kilopounds.

when seated deep, as long as it doesn't compress the powder (as that IS known to alter the burning characteristics) the bullet is not an obstruction, and moves very easily.

When it hits the rifling, pressures rise as it overcomes the resistance of bullet/rifling/bore.

a bullet that has a running start requires far less pressure to force it into the bore. I believe P.O.Ackley proved that out.

now jam that bullet INTO the bore, and you have to overcome the rifling with much greater force.

I used to know what/why this was. Modulus of elasticity or something else with a foreign sounding name.

I believe it is the same theory is behind breaking a thread with your hand... pull it slow and it will cut to the bone. Give it a snap and it will break easy.
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  #10  
Old 01-19-2013, 10:02 AM
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Re: Chamber pressures

I'm not gonna build a pyramid to qualify my points. I was simply focused on seating with all else allowing this single comparison. That is in a sense, shallow seating due to one barrel chambered with a long throat -vs- deeper seating with another barrel chambered to a shallower throat. Same cartridge, powder amount, bullet, bullet coatings, counter adjusted to same neck tension, same throat clearances, leade angles, trim clearance, neck clearance, etc.
Maybe this departed from the actual context of the question. Or maybe it should have been further qualified.

With seating adjustments off the lands there truly is a mountain of combining factors that could lead to MV wiggling a bit up or down -without every qualifier taken into account.
And I agree that longer freebores change the ballgame, but that wasn't in question right?

Quote:
Originally Posted by nanuk-O-da-Nort View Post
regarding chamber pressures, I think of it this way, the bullet has little to do with pressure until it encounters the rifling.
You mean bullet SEATING has little affect on MVs until touching rifling right?
If neck tension was normalized in comparison, this could be true.
If your suggesting that the bullet MASS doesn't matter until hitting rifling -because it's relatively low, this is totally wrong.

I have no agenda here to scare anyone from deep seating. To me this seems only significant to consider when nearing a donut.
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  #11  
Old 01-19-2013, 04:46 PM
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Re: Chamber pressures

I can't 'prove' it but I really doubt any dangerous 'donut' could form in any normal cartridge before a head seperation would occur.
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  #12  
Old 01-19-2013, 06:30 PM
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Re: Chamber pressures

Quote:
Originally Posted by Mikecr View Post
....That is in a sense, shallow seating due to one barrel chambered with a long throat -vs- deeper seating with another barrel chambered to a shallower throat. ... I interpreted the OP's ? as being in the same gun, different seating

And I agree that longer freebores change the ballgame, but that wasn't in question right? Right... same gun, same chamber.


You mean bullet SEATING has little affect on MVs until touching rifling right?
No, we are talking about pressure, and lower pressure = lower MV, all else being equal.

If neck tension was normalized in comparison, this could be true. I have never found neck tension to affect Pressure/MV that much. When a bullet is seated in a tight neck, it resizes the neck, so the neck is only so tight, or getting less tight, but cannot get more tight. I believe this is why some revolver cartridges require a very deep crimp, to continue to impede the bullet after case expansion.


If your suggesting that the bullet MASS doesn't matter until hitting rifling -because it's relatively low, this is totally wrong. If a 270Win is chambered in a 30-06 with a 110gr bullet, and fired, pressure will be XXX. If the 270 contained a 160gr bullet, I believe, and have read other data that supports, the pressure would be XXX + a miniscule amount.

I have no agenda here to scare anyone from deep seating. To me this seems only significant to consider when nearing a donut. Agreed, Donuts, no matter what creates them can cause an issue, not because of the donut, but because of the neck not having room to release, if the donut causes a solid contact in the chamber neck. In a very loose neck, donuts do no harm.
Quote:
Originally Posted by big wally View Post
..... I once read an article on chamber pressures with the different pressures for a deep loaded bullet and a long loaded bullet that touched the lands but cant find it now can any of you recolect this article or any that might be relivent....
I made an assumption that Wally was talking about bottlenecked cartridges here. I believe Straight walled cases offer a different scenario.
Wally
Quote:
Originally Posted by Mikecr View Post
If you're talking reasonable seating adjustments, deeper seating(by itself) increases pressure a relatively small amount due to reducing case capacity and increasing load density. It was ONLY this statement that I took some exception to, as I had read different, and with a couple minutes of Google, I found more than the one site, by someone I would call reputable on pressures, that had conflicting info. I had put it out there not to argue or to call you down, only to show that someone disagreed with you.

I apologize, if you took offense to that.
I have been perusing numerous posts on the long range accuracy stuff, to help me with some development, and I hope you continue to post, and don't "Ignore" me, in the event I have questions that you can answer.
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  #13  
Old 01-19-2013, 11:25 PM
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Re: Chamber pressures

As both Mikecr and Hornady pointed out the bullet starting at or in the lands will create the highest pressure....In other words they both agree then that backing off from the lands will reduce pressure.....
We all have to agree that at some point pressure will start rising because of the burning characteristics of a given powder and case volume..Obviously this is not a linear curve but more of a reverse bell...
Because these said increases are small and for the most part insignificant and often are not valid in the range most bullets are seated...I believe Hornady chose to illustrate only the obvious scenarios to further impress the danger aspect of high pressure and root causes....
Safety is the key issue in reloading and if a newcomer asks for advice it is probably a good idea to feed him something easy to digest...
IMHO,
Randy
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  #14  
Old 01-20-2013, 06:45 AM
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Re: Chamber pressures

Yes i was talking about bottleneck cases namely 17A/H and 22"K"Hornet,
Althouh i have been a re-loader for about 30 years have not gone that deep into it i load very conservitly I started with a 222 then a 22H then a 243 back to hornet and nown 17A/H and thinking about a 22"K" now.
My 17A/H is a Ruger 77/22H ALL WEATHER that i changed to a switch barrel gun.
When i was using the standered 22H my best accuracy was when loaded to touch with 35V/Max at COAL of 1.830" but the magazine took a max of 1.808" but for proper feeding 1.8"was best
I now want to use the 22H barrel again as a (K,d )hornet so i am looking at ways to shorten the freebore to alow me to load to touch and magazine fed rounds
I canot touch the reciever or bolt cause that is set up for NIL head/rim space with my 17 /H
I hope to remove some of the shoulder on the barrel to alow me to screw it in a complete turn to alow the extractor slot to line up and then remove metal from the chamber face to set up my rim space and of course this will shorten the case length ( about 60 thou i think) so hopefully i can get a smith to use a (K )ream to put a normal length chamber in without putting a Ruger lenth freebore in.
Hopefully this will give me a rifle that takes a factory round to blow out to K Hornet and alow me to load from the magazine whilst loading out to touch.

You lads have a lot more experiance than me amd as i dont have a proper reliable smith within 100 miles of me i need to get idea,s off you
Do you think it will work.

Wally
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