DaveTooley
Gold $$ Contributor
Yes it does. Bolt thrust increases as well as the hoop strength of the barrel tenon.
How to open a bolt face?
- Thread starter haeffnkr
- Start date
The same reason why you can shoot a 223 out of a Thompson Contender, But not a 308 win. Both operate at around the same pressures.
Fast14riot
Gold $$ Contributor
Its that s.i. part of p.s.i. that causes the problems with opening up the bolt face.
Bolt thrust is easy to calculate:
Bolt thrust - Wikipedia
It's likely as others have stated, that the mini receiver was not designed to handle the bolt thrust of "standard" diameter caseheads.
Other receivers, like the ubiquitous M700, use the same receiver design insofar as bolt lugs and abutments whether it's a non-magnum, or magnum- which is why opening the boltface or changing bolts is commonly done. Different story when it comes to Lapua boltface cartridges, where the increased bolt thrust is not considered in the design engineering. Some will do this conversion on standard magnum receivers (not moi), and while it's not likely to result in a catastrophic failure off the bat, the potential for plastic deformation of the lugs and abutments (setback) resulting in increasing headspace is expected.
Where does one find these hoop strength and bolt thrust calculations? Ottesons books?
Thanks.Bolt thrust is easy to calculate:
Bolt thrust - Wikipedia
en.wikipedia.org
It's likely as others have stated, that the mini receiver was not designed to handle the bolt thrust of "standard" diameter caseheads.
Other receivers, like the ubiquitous M700, use the same receiver design insofar as bolt lugs and abutments whether it's a non-magnum, or magnum- which is why opening the boltface or changing bolts is commonly done. Different story when it comes to Lapua boltface cartridges, where the increased bolt thrust is not considered in the design engineering. Some will do this conversion on standard magnum receivers (not moi), and while it's not likely to result in a catastrophic failure off the bat, the potential for plastic deformation of the lugs and abutments (setback) resulting in increasing headspace is expected.
Interesting. The larger the case head diameter, the more force (thrust). Opposite of something like surface area of tires on the roadway or the size of the pad under a crane outrigger. Learn something every day!
Best explanation here. Scroll down to the thrust section.
A Look at Bolt Lug Strength - Lilja
All action designers are concerned primarily with producing a safe and functional product. All other considerations such as weight, finish, eye appeal and other cosmetics are secondary. From a safety standpoint, the strength of the bolt lugs is of prime concern. Bolt lug shear strength depends...
DaveTooley
Gold $$ Contributor
I have to respectfully disagree with that. Think about case head separation. It can't be both ways.
I have never bought into the idea that a roughened up chamber lessons bolt thrust. Possibly reducing the brass flow into the shoulder area maybe.
Not following, Dave...
If there was zero adhesion of the brass to the chamber wall, wouldn't the entire case slide back to the boltface at ignition- preventing casehead separation (blowing out the shoulder instead)?
IOW, isn't it the case sticking to the chamber that causes the case head to separate in a rifle with excessive headspace?
From the WIki bolt thrust page...
Friction effects
A complicating matter regarding bolt thrust is that a cartridge case expands and deforms under high pressure and starts to "stick" to the chamber. This "friction-effect" can be accounted for with finite elements calculations on a computer, but it is a lot of specialized work and generally not worth the trouble.<a href="https://en.wikipedia.org/wiki/Bolt_thrust#cite_note-2"><span>[</span>2<span>]</span></a>By oiling proof rounds during NATO EPVAT testing procedures, NATO test centers intentionally lower case friction to promote high bolt thrust levels.
Please explain what I'm not understanding here?
Happy Thanksgiving, everyone!
jackieschmidt
Gold $$ Contributor
I leave my chambers with the finish that the reamer leaves.
DaveTooley
Gold $$ Contributor
I'm not saying that there is no adhesion of the case in the chamber but it has little influence on bolt thrust. The case is driven forward by the firing pin. Pressure builds, when the pressure exceeds the yield point of the brass the case expands in every direction. using the case head separation line, as an indicator, shows that part of the case, about .400" + up the case moves to the rear to fill the chamber. That happens each and every time the case is fired regardless of the chamber finish, regardless of shoulder bump. If it didn't why would we need to bump the shoulder when sizing? Varying degrees of hard bolt lift is an example of bolt thrust.
" IOW, isn't it the case sticking to the chamber that causes the case head to separate in a rifle with excessive headspace?"
That would be partial case adhesion with pressure pushing the rear part case against the bolt face. Thus bolt thrust. Can we manipulate the bolt thrust numbers one way or the other in a small way. Maybe
" IOW, isn't it the case sticking to the chamber that causes the case head to separate in a rifle with excessive headspace?"
That would be partial case adhesion with pressure pushing the rear part case against the bolt face. Thus bolt thrust. Can we manipulate the bolt thrust numbers one way or the other in a small way. Maybe
On top of that, the amount of deflection in a strong, front locking action, is pretty insignificant; under .001". It doesn't take much pressure to move the case head .001". I made a fixture which was essentially a chamber section, split at 60 degrees. I can the clamp a case in it by putting it in the three-jaw chuck. It has about 1/2 inch of protrusion on the case. I can put a rod through the headstock and push on the inside of the case. With a dial indicator, I can tell how much I have to push to deflect that case head by .001 or more. I don't think it is much over 200-300 pounds. It varies according to where the case is gripped.
Now, in an action which is not so rigid, like a Lee Enfield, the bolt will deflect about .004 or .005 when fired with heavy loads. If the case adheres to the chamber wall, case head separation will occur in 4 or 5 firings. If the case does not adhere, the shoulder will move forward that much until the action will no longer stretch enough to accommodate the case. I have tested this. I even made up a fixture, using a lead ball, to measure the amount of deflection of the bolt head. It was a bit of a failure because I couldn't separate bolt deflection from receiver stretch. I might try again later. First things first though. First, I have to achieve a respectable result at the Tack Driver! WH
Now, in an action which is not so rigid, like a Lee Enfield, the bolt will deflect about .004 or .005 when fired with heavy loads. If the case adheres to the chamber wall, case head separation will occur in 4 or 5 firings. If the case does not adhere, the shoulder will move forward that much until the action will no longer stretch enough to accommodate the case. I have tested this. I even made up a fixture, using a lead ball, to measure the amount of deflection of the bolt head. It was a bit of a failure because I couldn't separate bolt deflection from receiver stretch. I might try again later. First things first though. First, I have to achieve a respectable result at the Tack Driver! WH
And …. Don’t forget that today modern commercial cases can be steel , coated steel or brass or coated brass ( which was around commercially for a while . Each has different reactions to expansion and adhering to the chamber walls . Then we can get into a situation where we are marginal on thrust pressures under normal conditions but when things change , like wet cartridges, it could become a bit too much bolt thrust and ruin a good day hunting with stuck case , broken extractor or a locked action .
WSnyder
Silver $$ Contributor
Engineers Toolbox has a pressure vessel calculator. It’s typically used for things like tubes in a boiler.
It’s not intuitive and not simply plug and play without some research and understanding about what is going on.
Mby if I get the time I’ll run the numbers of a PPC and BR case.
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