Interesting failure NRA-UK report

[savagebrother]

Hey guys your right about slow powders and over pressures with mild or light loads.
Do you guys remember when savage came out wth there smokeless muzzle loaders???
Well I got two of the original or ML1 which used a module to hold the 209 primer used for ignition, I knew savages PR guy at the time and I heard thru the grape vine they were going to release them and upon qestioning him he said want to work to test loads??
I was more than happy too.
Well we who were in on the beginning did about 5 years worth of R&D on different powders and there loading screen and even did pressure reads on loads.
We found some very interesting things happen, as I tell you our findings you will understand where a very reputable bullet, brass, and ammo manufacturer developed, or should I say bought the rights to a type of loading screen that give much higher velocities over standardized loads without high pressures and heat.
Well we first started using fast powders: 5744, 4759, lil gun, 4227 in .50 cal. Gun. 4227 was listed last because it was always hard to ignite and gave poor accuracy in most guns
Now guys started to thinking what about slower powders ?
Well we started to look at loads for cartridge guns and trying them
Imr4198 and h4198 were tried but they were very erratic with accuracy. Loads for these were 300 grain hornady XTP's with sabot
And a favorite load of 5744 was 40-45 grains by weight. This was very accurate and recoil was light but this was the limit load wise for the faster powders but it did exactly what the inventor of this system wanted - a light recoiling muzzleloader with good killing power for his wife,nlittle did he know he had opened pandora station box!!
Well we started to look closer at what was happening??
I don't remember who did this first but they took a weighted amount of fine sugar and poured it in and then there then seated a saboteur bulle, 300 gr. XTP. Then loaded up a 209 primer and fired it. It moved 1-1/2" to 2-1/2" up the bore !

[savagebrother]

With just the 209 primer!!! Well we realized that the moment of ignition our chamber, the space between the saboted bullet and the breech plug was growing as the bullet began to move. Now right off we found we needed a very tight fitting sabot. If you load smokeless powder load with a loose or light fitting sabot 99 out 100 times it will not fire. Smokeless powder needs confinement and pressure to burn at optimum rates. I'm sure all of you have lit a pile of smokeless and watched it fizzle. We started loading bigger and bigger loads of slower powder. We found a sweet spot at about 20 % above what was a standard load. We had to compensate for the ever expanding chamber. This led us into a new direction, duplex loads !!!! Ahhhh!!! Duplex loads??? Yes, when some of the guys started using heavy loads of slower powder they noticed two pressure spikes milliseconds apart. Some actually got bulged barrels at 1-/12" to 2-1/2" in front of the saboted bullet, does that distance sound familiar??? Yeah well that's also where the second pressure spike occurred. This is where duplex get comes in. Remember slow powders need long barrels and tight twist rifling to do their thing. What happened is for a given load of say 75 grains by weight 15 grains of that would be 4759 and the bulk 60 grains would be 4198. What this does is bring the slow powder up to optimum pressure quicker, this made for great accuracy and great velocities. No any company that recently came out with ammo in standard calibers shooting anywhere from 100 to 200 FPS without causing over pressure signs in shooting these loads??? The guy that put all this research together was a good friend of mine and I know he was selling all his info to a reputable manufacturer. Unfortunately he passed away before I knew any more about it.
Shortly after he passed superamunition was released. I think you know who I'm talking about. Now the 30-40 krag has a long throat for the 220 gr bullet.

[savagebrother]

The krag rifles have a long throat meaning free bore that has no rifling. This space causes the two pressure spikes in a pressure reading. So a rifles ability to hold a given pressure for a given amount of time. Time being the key measure here. The bullet jumps the lead or throat of the chamber to the rifling where it now is graving into the rifling causing it to slow momentarily, thus our second pressure spike. With normal loads to loads in the high to maximum range you probably won't notice much if anything and the spikes will be close to the same pressure. Now back to time, if you try a load that is at or below standard loads the time at pressure now jumps even doubles and where is it at, right in front of the lead or free bore and the burning powder behind it now jumps to full pressure and it has a pressure wave in its way!!!
The rifle then goes boom.
SB

[BM1455]

Butlersrangers, that Swiss blowup kind of messes with my theory. It would be interesting to see a high speed video of each of the blow ups. After the high pressure event they are not coming apart as I would have expected.
Just a couple of notes on the US Krag I just rebarreled:
The bolt did have a lot of over travel with no case in the chamber. If this is the same with a 6.5X55, the bolt could follow a severely resized case way to far.
The chamber was .010 to .012 larger than an unfired case. Not sure if this was from an oversize reamer when the gun was first assembled or if the barrel swelled with use over the last century. If it swelled with the relatively low pressure loads then the barrel steel is fairly soft.

The steel on Norwegian Krags is like the steel used in SE Mausers. It was probably some of the best in the world at that time.

[butlersrangers]

Bump - because it is relevant to a recent KCA thread.