What exactly causes the bolt to jam?

[barbarian]

Greetings everyone.

A question here for those with more knowledge than me.

Let’s say you are doing load development for your new gun and you work your way up to a point where you get too much pressure and the bolt jams. What exactly has happened that causes the jam ?

I know it’s too much pressure, but what has physically changed that caused the hard bolt movement ? Did the brass expand lengthwise from the shoulder to the bolt face and its holding force against the bolt ? Just wondering “exactly” why it jammed.

Yes there can be many other reasons for a bolt jams, but purely for this conversation pretend it is simply over pressure.

Thanks very much.

 

[Beeron]

Bolt jam as evidenced by sticky or hard to lift bolt?

Ive had that happen from a crappy chamber ream on a howa 1500.

Not a pressure issue at all.

Case expanded more in the forward part of the chamber than in the rear either due to chamber irregularities or variable case expansion.

Primary extraction on bolt lift was inhibited but possible but i could not drive the bolt back without a rubber mallet (having bruised the heel of my hand beyond use).

Im no expert but one variable as I understand it is where primary extraction occurs in the cycle, if at all.

 

[wesson357]

Brass can flow in the bolt face ejector hole or slot making it hard to rotation. It will make "ejector swipe" mark.
The brass can as well expanded pass it's "spring back" limit by the over pressure and not release it's grip in the chamber.

 

[guntech]

Greetings everyone.

A question here for those with more knowledge than me.

Let’s say you are doing load development for your new gun and you work your way up to a point where you get too much pressure and the bolt jams. What exactly has happened that causes the jam ?

I know it’s too much pressure, but what has physically changed that caused the hard bolt movement ? Did the brass expand lengthwise from the shoulder to the bolt face and its holding force against the bolt ? Just wondering “exactly” why it jammed.

Yes there can be many other reasons for a bolt jams, but purely for this conversation pretend it is simply over pressure.

Thanks very much.

The rear case has expanded and now is slighlty larger than the chamber after firing. The case has also expanded lengthwise as much as possible and is now slightly longer than the chamber, so you have a very tight locking lug to locking recess fit. Very easy to gall a lug now. That's why it is reccomended to grease lugs.

Don't load this hot as there is nothing but headaches at this pressure level.

 

[SniperDon]

Greetings everyone.

A question here for those with more knowledge than me.

Let’s say you are doing load development for your new gun and you work your way up to a point where you get too much pressure and the bolt jams. What exactly has happened that causes the jam ?

I know it’s too much pressure, but what has physically changed that caused the hard bolt movement ? Did the brass expand lengthwise from the shoulder to the bolt face and its holding force against the bolt ? Just wondering “exactly” why it jammed.

Yes there can be many other reasons for a bolt jams, but purely for this conversation pretend it is simply over pressure.

Thanks very much.

Well you know they say pressure bust pipe and cause the jam!

 

[Lee Enfield]

Too many variables. As Guntech referenced, materials can be a problem.
Often if the bolt and receiver are the same hardness it reaults in galling.

I had a stainless Ruger that the bolt would gall (stick) to the receiver if I ran it fast... and that was closing the bolt, i had to beat the thing open more than once with a partially chambered round.

My favorite gunsmith pointed out that the first stainless guns were notorious for it - thats the major reason that stainless S&W revolvers had hard chromed hammers and triggers very shortly after debut.

 

[Mysticprecision]

The load has exceeded the elastic limit of the brass case. Case expands and doesn't contract, wedging itself into the bolt face and chamber. That force pushes the bolt back into the receiver locking up the various contact points making it very hard for a human to rotate the bolt body.

Jerry