Barrel pressures

[FalconFN]

I've just picked up an OU and got a very scruffy Spanish sxs with it, the proof marks on the OU say 1200 kg but the sxs is stamped with 850 kgs. They are both 70 mm chambered but am I correct to think that most 70 mm shells are around 900 kg? I'm not going to keep the sxs but im intrigued as to which shells are suitable for it?

Edited October 27, 2015 by FalconFN

[Guest cookoff013]

70mm ones.

[FalconFN]

Are they not 900 kg cm² then? I can't find a definitive answer online as some seem to say that 900 BAR is standard for nitro proof pressure (917 kg cm²) for 70 m shells and others suggest carts are as low as 650 BAR. Would it be better to stick to 65 mm carts?

[neutron619]

What cookoff is trying to say in a very concise way is that if you have a nitro-proved shotgun with 70mm chambers (you do) then you can shoot any cartridge with a fired length of up to 70mm. It's as simple as that.

 

To explain why you shouldn't care about pressures:

 

There are at least three pressures (if not four) used for proofing shotguns and their ammunition, all of which have been put on barrels as an indication of the proof test performed. Translating from the unintelligible names the proof house will give them, these roughly equate to the "normal working pressure", the "peak maximum proof pressure" and the "overall proof pressure". The possible fourth number is the "peak normal working pressure".

 

To complicate matters further, different pressures and units have been used for all of these numbers over time and different geographies.

 

To go one step beyond that, the pressures for each gauge and chamber length are different - a 3" .410 is proofed to a higher pressure than a 3" 12 gauge, and so on. (If you've ever noticed that modern .410's have very thick chamber walls, this is why.)

 

You can sometimes work out which of these various numbers is stamped on the barrel by a combination of the proof marks, knowing where the gun was proofed and when and a bit of logical deduction, but it's not always possible and you shouldn't trust a guess on that basis.

 

The only safe assumption to make is to find the proof mark that gives the gauge and chamber length (i.e. "12-70" - often in a diamond for Spanish-proofed guns) and use cartridges of that (fired) length or shorter.

 

Due to the increasingly litigious nature of our society, you will find that all modern cartridges are well within the lowest of any quoted pressures for the gauge / chamber length specified - the cartridge companies cannot be sure you aren't going to put one of their shells into a 100-year-old shotgun originally designed for black powder and updated with nitro proof later, so they have to operate well within the bounds of safety.

 

PS: Cartridges may say that they operate within an 850kg or 900kg proof, since that's what's tested, but they actually produce pressures far lower than that - for a normal 12 gauge shell (i.e. not 3"/3½") less than 740kg/cm³ (ordinary working pressure). Most clay shells are half of that and only the fastest heavy game loads get close to it.

Edited October 27, 2015 by neutron619

[FalconFN]

Thank you neutron for the full and thorough answer - and cookoff for the concise version. I've still got lots to learn.

[Stonepark]

850kg (12000psi) is fine for 70mm up to 1 1/2oz as in order to meet cip approval, cartridges in 70mm or less max out at 740kg (about 10500 psi).

 

Cartridges will on average generate 8000-9000psi, unless you are running a hotter loading.

 

Your gun will need something in the range of 40,000psi to burst the barrel if in good condition.