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 Post subject: Breech pressure?
PostPosted: Thu Dec 13, 2012 10:14 am 
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Joined: Fri Nov 16, 2012 9:58 am
Posts: 30
So, getting into reloading hot and heavy but I'm having a hard time understanding exactly what breech pressure is? Is it pressure the hull can withstand? Is it the pressure the gun can withstand? I searched a little, 4/5 pages back and found nothing? Does pressure differ in different hulls/guns? What's safe for semis or O/U? I will be reloading Remington hulls if that helps.

Also, when we go to meat shoots at my local club and guys go into shoot offs and they are standing back past the end of the sidewalks, are those guys using "hotter" loads, cause that's a darn long ways? Any inputs great! Thanks guys




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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 10:52 am 
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Joined: Thu Aug 31, 2006 10:06 am
Posts: 2263
Location: UK, England, Britain
pressure, is pressure. use a respected and published recipe from a manufacturer or media.

i once had a disturbing conversation with a reloading friend, who discussed back pressure, frontal pressure,sidewall pressure and chamber / head pressure. he is talking out of his arse.

find a decent recipe, then reload.


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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 10:54 am 
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The hull is only a container that holds all the components in place and seals the breech end of the barrel, the hull cannot withstand the breech pressure out side the firearm, it needs the chamber to properly support it. Without the chamber there is no breech pressure. If you should light off a shotshell outside the chamber it is fairly benign, as the hull is weak and the powder will not burn fast enough to develop any real pressure. You can prove that to yourself by taking a charge of smokeless powder outdoors, putting it in a small pile and lighting it. While it does flare up, it is pretty unremarkable. Smokeless powder only burns vigorously enough to create pressure if it is contained, as it is in the chamber of a shotgun.

A 12 Gauge shotgun with 2 3/4 or 3 inch chamber(s) are designed for a maximum average service chamber pressure (or breech pressure) of 11,500 PSI. 12 Gauge shotguns with 2 3/4 or 3 inch chambers are proof tested @ about 18,000 PSI. Modern U.S.made shotguns, regardless of action type are proofed at the same levels. Most imported shotguns, Beretta, Benelli, Franchi, Perazzi, Kreighoff etc, are made to very similar service and proof standards, regardless of action type.

Each gauge is designed to handle different chamber pressures. The 12 Gauge is unique as it has different pressures for 2 3/4" & 3" at 11,500 PSI and the 3 1/2" chamber has its maximum average service pressure at 14,000 PSI.

Lymans manual explains it or you can go online to the Sporting Arms Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) website.

There is too much made of chamber pressures on this forum as most folks believe that 7,000 PSI is somehow safer or better than 11,000 PSI. Not true!!! They are both safe and the truth be known, you should opt for the 11,000 PSI load, because pressure is your friend.

Any data that you get from the powder manufacturers or the legitimate component manufacturers stick to SAAMI standards, so any safety concerns are really not valid, as long as you stick to the data as it is presented.

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Last edited by dogchaser37 on Thu Dec 13, 2012 11:55 am, edited 1 time in total.

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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 11:26 am 
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Joined: Fri Nov 16, 2012 9:58 am
Posts: 30
Thanks dog! That answers a lot! But how come some loads show over the 11500? Are they safe? I just picture actions blowing to pieces on the groung!


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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 11:36 am 
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Location: E. Rochester NY
Maximum pressure for loads is currently measured as "PSI" for modern loads, on a transducer attached to a firearm's or test barrel's breech, and read electronically, usually on an oscilloscope or similar. It is generally listed as the maximum pressure observed during the firing sequence of the cartridge in the particular test barrel.

Actually, pressure curves can be shown for the entire sequence of firing, and they are quite interesting to ballisticians.

The older readings were taken as Lead Units of Pressure (LUP) or sometimes Copper Units of pressure (CUP) , which used a pressure gun and a chamber to measure deformation on a lead (or copper) cylinder.

The PSI and the LUP readings are NOT necessarily related or compare to each other in any meaningful way.

Barrels are TESTED with overloads ("proof" loads) with loads much higher than SAAMI maximums.

You have to understand, too, that different gauges of shotguns have different SAAMI maximum working pressure.

Rifle and pistols are designed to take maximum pressures significantly higher than shotguns, as well.

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Last edited by BobK on Thu Dec 13, 2012 11:42 am, edited 1 time in total.

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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 11:40 am 
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What loads and what gauge are over 11,500 PSI?

And the answer is no, to blowing up your shotgun over a few hundred or even a few thousand PSI. Shotguns usually blow up because there is an obstruction in the barrel. It would take a heck of a reloading mistake to destroy a shotgun. You can break some parts in semi-autos if you are over pressure, but that is not the same as blowing a shotgun up.

Think of it this way, you are putting your face on the stock of a shotgun, inches away from the breech. Would you really put that shotgun up to your face if it would self destruct at 13,000 PSI???? Of course not!!!!!

11,500 MAP for a 12 gauge is the average service pressure that the gun is designed to handle for 100's of thousands of rounds, it is not the maximum pressure that it will withstand. Shotguns are proofed at 18,000 + PSI and they don't self destruct at that pressure either. The proof of that, is you bought the shotgun.

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Last edited by dogchaser37 on Thu Dec 13, 2012 11:45 am, edited 1 time in total.

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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 11:44 am 
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I won't argue with Dog when it comes to modern guns, but - a low pressure, say under 7000psi is nice and recommended for the " old ' guns with 100+ year old stocks. Also most guys who reload for damascus barrel guns like the 7000 range. A side benifit is low pressure loads mean less pressure on the rear of the shot column so there'll be less deformation and better patterns. I have over 10 SxS's made in the 1800's with damascus barrels and shoot different ones three times a week at clays. I like pressures, or less, that were intended to be shot in them. A nice 3/4 or 7/8oz load at 1150fps and around 6500psi doesn't recoil, cheap to load, and easy on that old wood stock.


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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 11:49 am 
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bladesmith,

You are confusing chamber pressure with recoil, they aren't the same, and they don't have the same effect on old wood. What you want is a light payload and lower velocities for less recoil with old wood, chamber pressure has nothing to do with it.


Lots of misinformation out there about chamber pressure and that includes shotguns with damascus barrels and old wood.

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 Post subject: Re: Breech pressure?
PostPosted: Thu Dec 13, 2012 1:28 pm 
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Randy Wakeman has a fairly easy to follow discussion of pressure here (including the SAAMI specs):

http://randywakeman.com/shotgun_pressures.htm

All modern guns auto, pump, o/u have to meet these specs. The SAAMI peak pressure for the 3-1/2" 12 gauge does go over the 11,500 psi peak pressure (14,000 psi actually)

The trap handicap guys do shoot heavy loads - but it's done by the correct powder choice - not necessarily by pushing pressures. They are looking for a powder/hull/primer/ combo that gives higher velocities for heavier payloads.

So long as you are within SAAMI limits you're fine. Most of us try to leave some margin on the 11,500 level in case of variations in loading. For example crimp depth can have a big impact on pressure, but it's not really discussed - (aim for 0.050 - 0.055") . I think if you load around 10,000 psi +/- in a modern gun you get a reliable performing load and reasonably clean burn.

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 Post subject: Re: Breech pressure?
PostPosted: Fri Dec 14, 2012 11:52 am 
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This is not exactly on point of the original thread, but autos operate off of gas port pressure which is about 8 or so inches down the barrel from the Breach.

Consequently, you can usually use a slightly slower burning powder at lower pressures to make an auto work quite well.

Obviously you can use a fast burning powder with 10,000 psi breach pressure and it will have plenty of residual pressure to make the auto work fine. But you can also use a slightly slower powder at 7,500-8,000 psi breach pressures and get the same port pressure as the much higher pressure load at the breach.


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 Post subject: Re: Breech pressure?
PostPosted: Fri Dec 14, 2012 12:18 pm 
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moocher wrote:
This is not exactly on point of the original thread, but autos operate off of gas port pressure which is about 8 or so inches down the barrel from the Breach.



This statement is not true for ALL gas operated Semi-Automatic shotguns. Unless "Or so inches" is quite a wide dimension.



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