Interesting failure NRA-UK report

[reincarnated]

At risk of appearing totally out to lunch, what is SEE?

[butlersrangers]

SEE = Secondary Explosive Effect (aka - Detonation).

If I understand the basic theory correctly, there is a partial burn of the powder charge, the projectile moves forward and hesitates or hangs up, a pressure wave and intense heat occur, the remainder of the powder charge explodes creating amazing rifle shattering pressures.

I believe the most accepted culprit loads are under-loads of slow burning propellants.

(FWIW - The UK 6.5 X 55mm Norwegian Krag rifle mishap involved a load using 140 grain bullet and 34.4 grains Accurate 2520. I don't think Accurate 2520 is a slow burning propellant and 34.4 grains is a bit above the 'starting load' in some tables).

[reincarnated]

Thanks, BR. My burn rate list says Accurate 2520 is just a smidge slower than IMR-4064 and a good bit faster than IMR-4320. Jacketed bullet loads in the 34 to 36 grain load range have been in use since the 1930s for US Krags and the '06. My guess is that something else was amiss. Those case necks did not look kosher. Bullets for a Carcano? IIRC, those run .266-.268 and come in boxes that say 6.5 mm.

That Lee Navy makes me want to cry. I spent the better part of 3 years trying to find a good one at a price I could afford. I quit looking when those accidents appeared on the net.

[kragluver]

"Slow" is relative here. The research that was done back in the '60's would indicate a SEE event can occur for powders at or slower than 3031. I wouldn't have used any of the powders mentioned here for reduced loads. There are powders specifically intended for such loads.

[FredC]

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.

[madsenshooter]

If I remember correctly that Swiss rifle had a bullet sized .314. If the chamber is like my K31, the fellow would had to have pounded on the bolt handle to get it closed. Even so, I doubt that alone was enough to make the pressure the round delivered, likely an overload behind that too large bullet. Not sure, but I think that fellow gave up shooting after his incident.

[FredC]

Would have ruined my day and probably give me the willies anytime I started pulling on a trigger for a long time after that.

[butlersrangers]

I believe the K-31 Swiss mishap involved a .314" cast bullet in a .308" bore.

IIRC - The owner had some suspicion he could have thrown a double charge.

[reincarnated]

My K-31 has a .305" groove diameter. Just shooting .308 bullets is risky.

[madsenshooter]

Not with .308 sized cast bullets. I'm close to you with a .3055 groove, .294 bore. Mine is a SIG made barrel and I have shot my cast bullets sized .311, but found them more accurate when sized to .308. I don't think the chamber neck is large enough to allow a case with a .314 sized bullet to chamber without banging on the handle. Well, I guess mine would be, I just measured the neck ID on some Graf/PRVI cases that I know were full pressure, neck expanding loads, they're .315-.316. I agree with you on .308 jacketed bullets with some K31s. If I'd been dumb enough to jump in with near max jacketed loads that I've found online, rather than start low, work up, I imagine I'd have some pretty high pressure loads. (Don't tell anyone, but I was that dumb once, had to pull down a bunch of rounds after getting pressure signs with the first two.) Not rifle destroying high, but higher than I care to go. I guess they're sorta like US Krags, bore/groove dimensions can and do vary. Ditto those 6.5 Swedish chambered Mausers and Krags, with the Krags generally running on the large size.