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Chamber Pressure -- Norwegian Krags and Current Loads from Old Site

U.S., Danish, and Norwegian
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Kevinofborg
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Chamber Pressure -- Norwegian Krags and Current Loads from Old Site

#1 Post by Kevinofborg » Sun Apr 29, 2018 2:21 pm

dnovo on Tue Oct 31, 2006 8:12 pm wrote:

Conventional 'wisdom' (including some threads at this and other military rifle sites) states that while the Norwegian Krag was loaded at first to the same specs as the Swedes in their 6.5X55 Mausers, that the Swedes later 'went hotter' and that shooting a Norwegian Krag with current commercial loads is dangerous.

Okay, but at the same time, a fair amount of other information on the Net says this is simply not so, and the Krag will in fact digest current commercial loads, especially the later 1912 models built well into the 'modern' era.

While I am new to this site, I have been collecting and shooting for most of my life (I'm 59) and prefer fact to 'common wisdom.' I have only found one reference in English to the original pressure specs for the Norwegian Krag, admittedly not at what I would consider an 'authoritative site' (Wikipedia) but one that cites a Norwegian source as follows:

"6.5x55 a 6.5 mm (0.256 in) rimless round. Most variations are loaded for a chamber pressure of 350 MPa (roughly 51000 lbf/in²). Early rounds, with a 10.1 grams (156 grains) long round nosed bullet (B-projectile) had a muzzle velocity of around 700 m/s (roughly 2300 ft/s), while later rounds with a 9 grams (139 grains) spitzer bullet (D-projectile) offered a muzzle velocity up to 870 m/s (2854 ft/s)."

Okay, so SAAMI specs for commercial 6.5X55 Swede is 46,000 cup with 30-40 Krag loaded at 40,000. Assuming that military loads and rifles are normally speced out a bit hotter than commercial loads, which must be made up with the likelihood of being stuffed into some worn out and 'ridden hard and put away wet' old rifle, then is a commercial 6.5 Swede really going to make one of my Norwegian Krags go BOOM?

I can load my own, and would normally light load any older rifle, but I keep hearing this 'Norwegian Krag load' litany and wonder if it is, as the Wikipedia notes and others say, simply a tall story repeated too often or is it fact.

Does anyone know what pressure the Norwegian Krags were proofed at? Will a commercial load blow up one in good shape (there are GO/N0-G0/FIELD gauges available in 6.5 Swede that I use in my Swede Mausers and that would be a good starting point) or is that simply a tall tale.

Is there any hard data on this one way or another? Any information would be appreciated. Dave

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popeye replied Sun Nov 12, 2006 7:32 pm

The 6.5x55 Swedish Mauser cartridge was jointly developed by Norway and Sweden, this cartridge was adopted by both countries as an official chambering in 1894. Originally both countries loaded and used essentially identical ammunition. Later, the Swedes modified dimensions and loaded to higher pressures for use in their Mauser rifles, while the Norwegians kept the original version for use in the Krag rifle.

The Norwegian Krag is intended for working pressures of only about 45,000 psi.


From, Cartridges of the World, 11th Edition, by Frank C. Barnes

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Malamute replied Sun Jan 07, 2007 10:06 am

Comparing the chamber pressure of the 6.5x55 and the 30-40 is not the only factor to consider. The head size of the cartridge has some bearing on the ability of the action to hold a given chamber pressure. In other words, the 30-40 has a larger base size than the 6.5x55, and will "push harder" (more bolt thrust) against the bolt with a given chamber pressure than a cartridge with a smaller base size.

This didnt make sense to me when I first heard it, figuring "chamber pressure is chamber pressure". Well, not exactly. Bolt thrust is dependant on the size of the area pushing against it. A gunsmith friend told me about it. Somebody wanted X cartridge chambered in a Contender, he thought he could do it, because the factory chambered abother cartridge of similar pressure in the gun. He called the factory about it tho, and they told him "NO!! The bolt thrust is greater because the head size is larger, DON'T do it." I don't know exactly how to convert chamber pressure of a given round to bolt thrust, but somebody here may know.

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Merle replied with quote Sat Mar 10, 2007 9:10 pm

Malamute wrote:
"Comparing the chamber pressure of the 6.5x55 and the 30-40 is not the only factor to consider. The head size of the cartridge has some bearing on the ability of the action to hold a given chamber pressure. In other words, the 30-40 has a larger base size than the 6.5x55, and will "push harder" (more bolt thrust) against the bolt with a given chamber pressure than a cartridge with a smaller base size.

This didn't make sense to me when I first heard it, figuring "chamber pressure is chamber pressure". Well, not exactly. Bolt thrust is dependent on the size of the area pushing against it. A gunsmith friend told me about it. Somebody wanted X cartridge chambered in a Contender, he thought he could do it, because the factory chambered another cartridge of similar pressure in the gun. He called the factory about it tho, and they told him "NO!! The bolt thrust is greater because the head size is larger, DON'T do it." I don't know exactly how to convert chamber pressure of a given round to bolt thrust, but somebody here may know."


It's fairly simple, in theory, as gas pressure will fill a vessel to equal pressure throughout its volume. You take the pressure in PSI times the area of the case head in square inches (area= pie R squared) to find the back-thrust in pounds. This is the case at the instant of firing.

EXAMPLE: your load generates 40,000 PSI and your cartridge has an area of 1 square inch = 40,000 pounds of back thrust - if the case area is only 1/2 square inch, the back thrust is 20,000 pounds. This is the reason that cartridges with equal pressure can not always be chambered in guns such as a T/C.

Keep in mind that a rifle chamber is not a perfect world. The pressure curve can be modified by several factors, which will impact the total back thrust delivered. The taper of the case body will have an effect, as a large taper will increase the "wedge" effect. Also, a rough chamber will grip the case harder, a greasy chamber will not, etc, etc, etc.

Lot's of room for variations, which can only be measured by a strain gage, which will measure pressure over an extended time. Peak pressure is not the only issue here.

Hope this helps, and doesn't confuse.

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mountainmarty replied Mon Jun 18, 2007 6:17 am

Thats some great discussion about hot loading the Krag. Proof testing firearms is an interesting subject. I'm not an expert but, I've read that proof test loads exceed the pressure curve that the gun was designed to operate. I don't know what % they overload the cartridge, but I think the proof load is used only once, if the rifle doesn't explode it gets sent down the line. OK , thats one time the rifle has endured excessive pressure before it even leaves the factory. Ammunition is loaded for safe operation in all firearms that are chambered for it . For example the 45/70 gov. is loaded down so we can't blow up a Trapdoor Springfield...8x57 was loaded down because of the different bore sizes in the early and later Mausers. Now a smart hand loader knows the 1886 Winchester or Marlin can handle a hot 45/70 load, same with the Mauser, if the smart handloader knows the bore dia. he can achieve decent velocities that exceed factory 8x57 ammo. The smart handloader also learns to work up a pet load making sure to look for signs of excessive pressure. Some people will ingnore some signs.....HHHmmm flat primer, but the bolt opened easy enough!! That's fine for some people, but how many rounds at this pressure can the rifle shoot safely? 93 ? 206? 6? Lets talk about the Krag...I own one myself. My Grandfather gave it to me over 20 years ago. That rifle started my intrest in surplus firearms. The Krag has One Locking Lug it's been proof tested once already, how many times in its long life has it experienced excessive pressure? How many times will it stand up to a hot load? Some Mausers have a third safety lug on the bolt, the Krag doesn't have a safety lug. The Krag rifle is a beatifully crafted rifle. I love mine dearly thanks to Grandpa it's in my safe keeping...One day it will belong to one of my Sons!!

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Ken in Iowa replied Mon Jun 18, 2007 6:17 am

Norwegian Krags have 2 locking lugs where the US Krag has one. The bolt handle on the Krag design is the safety lug.

I understand Dave's concerns. I believe I would ask this question directly to the ammo manufacturers.

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Jeff S. replied Mon Jun 18, 2007 6:37 am

Merle is right:

Bolt thrust = pressure x area.

So for a typical modern standard base ammo: P = 62,000 psi, (about 50,000 cup)

A = 0.473 ^2 x 3.14 / 4 = 0.176 sq inches

BT = 0.176 x 62,000 = 10,900 lbs

Some of you might say the correct area to use should be calculated using the internal diameter of the case, not the external that I used in the calculation above. I would argue, though that the brass acts as a thick incompressible liquid during the firing procedure, so would transfer the pressure as it it were a hydraulic fluid, since it flows so much more easily than the hardened steel of the bolt face and chamber.

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Lumberjack replied Thu Dec 03, 2009 9:29 am

In Norway we dont use full power loads for krags.

I have written something about it here:
viewtopic.php?f=33&t=85517 (Unable to recover this link from the wayback machine - Kevin)

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Re: Chamber Pressure -- Norwegian Krags and Current Loads from Old Site

#2 Post by Rapidrob » Sun Apr 29, 2018 3:25 pm

I do not download for any of my Krags. There is no need to. The steel is some of the best ever made. The rifle is stronger than it looks,while not a "magnum" rifle,it will handle any military loads made for this caliber rifle. I could not count the number of rounds I have fired over the decades with never any problems or broken parts or even one stuck case.
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Re: Chamber Pressure -- Norwegian Krags and Current Loads from Old Site

#3 Post by lloydiam » Sun Apr 29, 2018 11:50 pm

You can tell people that ALL day long Rob but NO one will believe you, I don't download for mine either & I have NEVER had a problem !...................

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Re: Chamber Pressure -- Norwegian Krags and Current Loads from Old Site

#4 Post by dezzertrat » Mon Apr 30, 2018 11:04 am

I believe the US Krag has two safety lugs. The first and strongest one is the guide rib on the bolt which misses the receiver by a few thousandths of an inch when the bolt is closed. The second is the bolt handle fitting into a slot in the receiver.

The Norwegians created a second locking lug for their Krags by lapping the forward locking lug until the guide rib made contact with the receiver. The same procedure can be used on a US Krag, but doing so increases the headspace. I don't see any need to do this, instead I keep my loads within published specs.

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