Pressure VS Breech Thrust

[nanuk-O-dah-Nort]

has anyone come up with sound numbers as to what the Handi SB2 is capable of handling?

According to some barrel/rifle manufacturers, Pressure is pressure, and breech thrust is paramount

so, if a 500S&W, with a base diameter of 0.525, at 60K MAP, gives 13031 Lbs

why are the belted mags, with a base of 0.513, at 65K MAP, which gives 13428 Lbs considered way over the top?

is 400 lbs really all that much to get excited over?


also, if the base diameter is less, has anyone chambered a wildcat on the belted mag?

Here, I'm thinking a .458American, stretched a bit to clean up the neck, due to the cheaper brass, and it would seem to be an easy rechamber to a 45/70, but would require some extractor work.

or a 11.2x60 Belted Schuler

an ejector, with a recessed tab would seem to be a pretty easy conversion.

[nanuk-O-dah-Nort]

I'll add, that a 45/70 loaded to 65k has less breech thrust than a 500 S&W

[Spanky]

I'll add, that a 45/70 loaded to 65k has less breech thrust than a 500 S&W

 
A 45-70 loaded to 65k would thrust your shoulder pretty good.
 
 
 
Spanky

[quickdtoo]

The difference in your two comparisons(500S&W vs belted mags) is the belted mags are bottle necked rounds which is of particular importance in the Handi action. I've worked with a 300WSM Handi, it has a .555" case head, even at mid level pressures(50kpsi) it would flex the action enough to leave a gap between the frame and standing breech after the shot, the brass lengthened to keep it that way and would not chamber flush afterwards. I was unable to achieve velocities any better than 30-06 with 150gr bullets using data supplied by Johan at AA.

Reduce the case head size and that pressure works fine, my 376 Steyr has a .506" case head, 56-57kpsi loads work fine. I've shot Barnes 45-70 300gr TSX-FBs using their data which runs to 55kpsi, no problems there either, I've not seen any data for the 500S&W that runs to the SAAMI MAP of 60kpsi, best I've seen is about 53kpsi which reduces the breech thrust some. You can pretty much consider the 11-12k lbs the practical limit IMO, 270 Win @ 65kpsi.

IMO the flexing is due to the small pin the latch pivots on(not the frame itself), if it were bigger, I don't think there would be as much flex. But I don't think there's room to make the pin bigger, it would require a heavier latch and hogging out the frame to accomodate it prolly wouldn't work. The 2008 Handi Mags would have had a stronger frame made of a tougher alloy and different heat treating among other changes according to Tim Looney when it was on the drawing boards, but that's as far as they got.

Tim

[nanuk-O-dah-Nort]

so what I am hearing, Tim, is that pressure is NOT pressure, and there is another force at work somewhere, when it comes to bottlenecks.

could we surmise it has to do with the expansion ratio?   a 22-06 to 65K would have a good chance of flexing the frame as the 300WinMag?

[bucmeister]

Doesn't the taper of the cartridge case wall have a good bit to do with the actual breech thrust placed on the action.  If comparing two cartridges with the same case head diameter and loaded to the same pressures, one straight walled and the other the typical tapered wall bottle neck case wouldn't there be more breech thrust from the tapered round?  I always thought the tapered case wall tended slide backwards in the chamber just due to the differences in the dynamics of the grip on the chamber wall of the straight versus tapered case. 

[quickdtoo]

Adding a shoulder and tapered case into the equation and breech thrust at a given pressure should increase, it certainly doesn't help. A straight walled round has the least breech thrust in my thinking(and Marshall Stanton's of Beartooth Bullets), consider the fact the straight walled rounds rarely need to be trimmed, yet bottle necked cases almost always need to be trimmed.

Tim

 
http://www.shootersforum.com/leverguns-their-cartridges-general/2137-nobody-has-375-win.html

You are right! *The case walls on either the .38-55 or .30-30 are thinner, hence the increased case capacity! *However, there isn't the slightest hazard using this brass in your .375 Winchester. * Keep in mind that the .375 Winchester is a straight walled case, and most of it's stress during the firing cycle is outward, against the case walls... not back against the bolt face! *(P.O. Ackley did exhaustive studies on this phenomonon and conclusively proved the point) *Where you might get into trouble with the thinner case is if there was a significant amount of back-thrust against the breech-bolt, and then the thinner web of the case head could possibly cause a rupture if there was enough rearward movement of the case from perhaps excessive head-space.  However since this is a straight-walled case, not a tapered bottle-neck design, we don't have that back-thrust against the bolt-face.   Consequently the forces of physics are working in our favor when utilizing a brass case of thinner construction, especially when we look at the following contributing factors entered into the equation.
I'm not alone in the practice of using this brass, and others such as Paco Kelley will advise the same. *You can get one of two results:

The same velocity load as with standard .375 Win brass, but with lower pressures.

Or, a higher velocity load as compared to standard .375 Win brass, but with equal pressures.

I might also interject that todays brass is far higher quality due to advances in metallurgy than it was in 1894-96 when the .30-30 and .32 Special were conceived, thus necessary brass thickness then and now are somewhat different to contain equal pressures. * All this being said, the .30-30 case of the 1990's will withstand more pressure, more reliably than the same case made even 70 years ago, due to our more refined and perfected metalurgical sciences.

The truth is that the .375 Win Brass case is over-engineered for its intended purpose. * Keep in mind that it was conceived in a litigation oriented product development climate... in other words; * make that sucker lawyer proof! * Forget ballistic engineering and balancing case capacity, necessary strength and cartridge design.... first make it litigatiion proof, then worry about making it shoot!

Well, I better stop before I step up on a real-life soap-box!

Hope this helps!

God Bless,

Marshall

[ibgp3]

"Forward thrust":



Sometimes extreme examples make concepts clearer.
In the .44 case at the top, "forward thrust" is easy, it equals 0. So the breech thrust is the piston thrust at the rear of the case.
In the second case, the .44 is necked down to .22; but because it is bottle necked, calculation and visualization of the effect of the pressure on the rifle is difficult.
In the third case the barrel has a 44 chamber and no outlet. (Don't try this at home  )
Calculation of the "forward thrust" is easy: it equals the piston thrust at the back of the case.
Does the "forward thrust" cancel the piston thrust? Nope, sorry, they combine; just as they do in a bottle necked case.
So the forward thrust plus the piston thrust equals the breech thrust.
...and the greater the difference between the diameter of the case and the diameter of the bore, the closer the forward thrust gets to equaling the piston thrust..


With the same pressure in the case,
....a 22br will produce more breech thrust than a 358 Winchester.

[ibgp3]

so what I am hearing, Tim, is that pressure is NOT pressure, and there is another force at work somewhere, when it comes to bottlenecks.

could we surmise it has to do with the expansion ratio?   a 22-06 to 65K would have a good chance of flexing the frame as the 300WinMag?

The piston pressure is dependent on the internal diameter of the case.
...the piston pressure in an 06-class case is about 80% of the 300winmag
...the forward thrust in a 22-06 is about 75% of the piston pressure. [75%(80%)=60%]
...the forward thrust in a 300winmag is about 70% of the piston pressure.
breech  thrust of 22-06 = 80% + 60% = 140% of 300winmag piston pressure.
breech thrust of 300winmag = 100% + 70% = 170% of 300winmag piston pressure.


With the same pressure in the chamber
a 22-06 will have about 80% of the breech thrust of the 300winmag.

[quickdtoo]

Thanks for the visualization!! 

Tim

[nanuk-O-dah-Nort]

ibgp3

thanks for this info, very interesting

this is the first time I've read anything about the shoulder diameter minus the bore diameter, adds to the breech thrust.

this requires some further inquiry, regarding what others have said.

off the top of my head, and after reading it about 5 times, I now understand what you are saying.  Still working on understanding the numbers, but that'll come....

[Bill3006]

ibgp3,


I don't think the "forward" thrust adds to the breech thrust, if you consider it as a pressure vessel. As others have mentioned, breech thrust is a function of the internal case head area and peak pressure. There is a good treatment of it here: http://en.wikipedia.org/wiki/Bolt_thrust.

[trotterlg]

Two things working here.  Forward thrust and breach thrust.  Forward thrust is calculated using the pressure and area of the sholder plus the amount of friction of the bullet in the barrel (which isn't much)  Breach thrust is calculated using the pressure and the area of the base of the cartridge not counting the rim if there is one.  If there is no sholder then there is very little forward thrust on the barrel but with a large sholder area there is a lot.  The stress on the frame comes from the forward thrust on the barrel where the hinge pin has to carry all that load.  You can argue that the friction of the case in the chamber reduces the breach and forward thrust on the barrel, but that is not easy to calculate because of all the valriables of chamber finish and lube on the cases, so it is best to assure you will get no help from the case friction or strength.  This is why Short magnum rounds are such a disaster for a Handi Rifle.  Larry 

[hoytcanon]

I've never been much good at gymnastics... on a mat or in my head. 

[gcrank1]

I guess I must say for all concerned,"DONT TRY THIS AT HOME, OR ANYWHERE ELSE"
A 'pressure vessel' laying unrestrained upon the ground with a slightly snuggish cap on either end will blow both off and the vessel stay in place. Tightly cap one end and you know what changes. Even use a reducer (to emulate the chamber shoulder) and a snug only cap and a tight cap on the other end and you know what happens only with higher gas velocity.
Any forward thrust is completely negated by the greater thrust rearward otherwise we would have no projectile velocity.