Freebore and overpressure

[hemiford]

Say you have a chamber with a lot of freebore, something like a Weatherby. Your chambered cartridges are "well short" of touching the lands.
Also let's say you properly trimmed your brass length, and have good neck clearance and known non-excessive neck tension, perhaps no neck crimp either.

What would be possible causes of overpressure / pressure spike at that point ?

A tight bore ?
Wrong (too fast) powder ?
Too little powder fill, or too highly compressed ?
Unclean, oily chamber ?
Cold temp load workup then shot on a hot day ?

What else ?

I'm trying to get a handle on what could cause an accidental and catastrophic pressure spike.

 

[Dmagna]

It could be everything you listed

Obstructed bore, wrong powder, wrong bullet, cold weld.

 

[baldhunter]

What cartridge,powder,bullet and brass?

 

[hemiford]

No particular cartridge, I'm generalizing, results will apply to all.

 

[YZ-80]

All things considered, as you listed, the first thing that comes to mind for me is reduced charges for certain powders.

 

[Mikecr]

Unclean, oily chamber ?

That doesn't cause over pressure.
Add primer change, and firing a max new brass load in fully fire formed cases.

 

[Elk Hunter 338]

What cartridge,powder,bullet and brass?

what he said

 

[Dmagna]

That doesn't cause over pressure.
Add primer change, and firing a max new brass load in fully fire formed cases.

Yes it does. I have had popped primers and blown out of pockets because I left oil in the chamber.

 

[del2les]

Yes it does. I have had popped primers and blown out of pockets because I left oil in the chamber.

Too much oil in the barrel "may" lead to issues when firing loads that are near max. Been there myself.

 

[MagnumManiac]

Only cause I know of directly is an overload with a temp sensitive powder.
My 375 Weatherby, using the correct powder, will not allow enough powder to detonate, but it will go high enough to crater primers and have sticky bolt lift. It has .500” of freebore and will digest 3 more grains than a standard 375 Weatherby throat chamber does.
Excessive pressure is a rare thing if the correct burn rate powder is used with the correct volume.
Compressed charges do not increase max pressure or make a load dangerous, doesn’t happen unless the compression is forcing the bullet out of the neck…

Cheers.

 

[243winxb]

There are a few posts of 257 Weatherby magnum having problems.
Link


Takes slow powder, standard primer, light for caliber bullets, little to no neck tension. Long free bores.

The bullet moves out of the case to soon. Acts like a pluged barrel. 2nd pressure spike, high pressure.

Slow Double based powders, with nitroglycerin, may detonate under certain conditions.

Most rifle actions will contain 100,000 + PSI. Catastrophic? I think no. The wrong powder, yes, maybe?

 

[MagnumManiac]

There are a few posts of 257 Weatherby magnum having problems.
Link


Takes slow powder, standard primer, light for caliber bullets, little to no neck tension. Long free bores.

The bullet moves out of the case to soon. Acts like a pluged barrel. 2nd pressure spike, high pressure.

Slow Double based powders, with nitroglycerin, may detonate under certain conditions.

Most rifle actions will contain 100,000 + PSI. Catastrophic? I think no. The wrong powder, yes, maybe?

View attachment 576708

This is under ignition, not excessive pressure. Powder has failed to ignite correctly, case hasn’t sealed the chamber and the escaping gas has dented the case. I had this happen with both 25-300WSM & 6.5-300WSM using H4831, IMR4831 and the worst was IMR7828.
Pressure trace showed very low pressure around 28,000psi.

Cheers.

 

[The Oregonian]

Carbon ring can do it.

 

[Lefty7mmstw]

An oily chamber won't cause overpressure on its own, but it can sure cause back thrust on the bolt to go through the roof. It's been used to proof actions before.