Why not .270Win?

[KeithB]

For any object the max rotational rate will be that which generates a hoop stress greater than the tensile strength of the material. Governs everything from bullets to planets. For all practical purposes it is probably less since internal flaws and defects can increase the internal stresses.

 

[Ian]

Ok ok I concede!

The hoop stress limits of the barrel steel will be exceeded in most cases before you get enough rotational velocity to poof a well-balanced cast bullet. Thin jackets skivved by the rifling and soft cores have a definite limit. I think I remmber Arne Brennan blowing up a few jacketed pistol bullets trying different things in the .45 Raptor.

 

[KeithB]

Yes. it may certainly be the case that in a real world application other factors may well prevent most bullets from achieving a rotational rate that would cause self destruction. And w/o running the numbers I have no idea what rotational rate would be required of an object of a given diameter and composition to go poof. But once that bullet is no longer constrained from expanding radially by the barrel it will be subjected to forces that will try to tear it apart at the surface where the highest stress is evident.

 

[fiver]

Keith has a handle on this... LOL

 

[Pistolero]

I wondered why the bullet seem to not blow up right at the muzzle but a good distance away from the muzzle, like 50 yds+
My hypothesis is that the extremely high velocity is providing aerodynamic heating, which substantially lowers
the tensile strength of metals. My load that were blowing up were in the 3800-3900 fps range, about Mach
3.6ish, and a LOT of aero heating there.
Another factor to consider.

Hornady had problems with plastic tips melting and losing ballistic coefficient, recently went to a polysulphide
high temp plastic. We used polysulphide at Union Carbide (a LONG time ago) to make autoclavable medical
devices because if it's high temp resistance.

Bill

 

[fiver]

I always just figured they were going stupid fast and that was how far they got before you seen it happen.
probably started sooner.
it just got that far before it poofed enough to see and there wasn't enough left to see anything any further away.

 

[Ian]

Heat probably IS a factor, just guessing. Even the heat from barrel friction takes time to penetrate the jacket; not much time but still it isn't instantaneous.

 

[Pistolero]

Stagnation temp at Mach 3.6 at sea level is ballpark 1100F. That is a constant, so bullet temp is
increasing, while the heat source (friction) is removed at barrel exit. Both are probably factors, but the
continuing 1100F applied temp has to be a factor. Exposure is short. A 100 yd shot at 3900 fps,
is 0.077 seconds to the target. Mine were blowing at perhaps 1/2 to 2/3 of that, say 0.05 sec, or
50 milliseconds. Very high conductivity between the hot air and the skin, extremely
high convection coefficient, at that speed.

I remember asking my father how fast his F-4 Phantom II could go. He said, "Mach 2.2" at altitude.
I asked about down low, and he said a lot slower. I asked why, and he explained that the aerodynamic
heating would weaken the wings at over about Mach 1.5-1.7 at low altitudes. Stagnation is in the
~300F range there, enough to start to make aluminum structures.....questionable. My first introduction
to aerodynamic heating at about age 12 or 14.

Bill

 

[Ian]

Well a quick look at the webz shows a chart where pure copper only retains 40% of its 70⁰ tensile strength at 900⁰F. That is a pretty stong indicator.

 

[Pistolero]

Yep, remember in a former life I was a stress analyst and sometimes things got hot. I had to get a
general grip on what metals do at temps. There are really good reason that turbine blades are made out
of nickel, chrome, and other really fancy ($$$$) stuff. I took a course in aeroeng, too, and a couple in
fluids. Lotta weird stuff happens if you go really fast. The rocket sled dudes make all their support
for test articles at Mach 5, 6 and higher out of railroad rails. Takes long enough to melt that the
test is over. Mach 6 stagnation is around 2000F.

http://image.slidesharecdn.com/highspeedaerodynamics-091209082849-phpapp02/95/high-speed-aerodynamics-57-728.jpg?cb=1260347465

They never worried about fatigue in the all welded Ti structure of the Blackbird. It was annealed every
flight, erasing any fatigue cycles.

 

[JWFilips]

I'm starting to think you guys are bordering on Theoretical Physics here! ( for awhile here I thought I was back with my Physics chums at Keystone Observatory when I was in my early 20's )

 

[Brad]

I'm starting to think you guys are bordering on Theoretical Physics here! ( for awhile here I thought I was back with my Physics chums at Keystone Observatory when I was in my early 20's )

Far worse than that Jim, they are engineers.

At least Bill talks to your shoes.

 

[RBHarter]

I'm not , but I've seen it all in play .

I've had a passion for aerodynamics since my early teens .
I got to drive down a "rehab" line for the SR71 and got to spend some time with a couple of ground crews about the time they were retired . The thermodynamics involved in just getting the thing to the in flight fueling so it could fly a mission was incredible whether it is looked at from flight or physics .

Is there a way to visit the video mentioned ? I'd like to see it . The mechanics of the ejecta stream and the transition out of it are of great interest to me . Physically the bullet is supersonic inside the barrel so the ejecta is also . The bullet is passing through the ejecta doesn't actually reach airspeed supersonic until probably about 2/3 of the way through it ..... relativity ?

 

[Bret4207]

I wondered why the bullet seem to not blow up right at the muzzle but a good distance away from the muzzle, like 50 yds+...


Bill

In the spirit of keeping the pot stirred, I will point out that a certain guy called Starmetal figured that out a decade or more back- the bullet as gaining speed and rpms after it left the barrel!!!

Stir, stir, stir

 

[Bret4207]

SR71- Kelly Johnson and the boys were geniuses! Of course a relatively unlimited budget helps a lot.

Discussions like this on this site give me the warm fuzzies. I don't have the background or training to understand a lot of it, but I get the general idea...I think. Makes me try to think in a new way. Thanks guys!

 

[RicinYakima]

In the spirit of keeping the pot stirred, I will point out that a certain guy called Starmetal figured that out a decade or more back- the bullet as gaining speed and rpms after it left the barrel!!!

Actually, Dr. Franklin Mann wrote about this in 1904. The bullet will continue to accelerate after leaving the barrel because it has mass. Newton's Law that it will continue to accelerate until outside forces (air resistance) start slowing it down. IIRC, his 32/40 bullets continued for about 3 feet. Rotation resistance is just about nothing, other than air against the lands marks as it slows below sub-sonic.

 

[Ian]

Which of Newton's laws was that? Acceleration will decay to zero and inertial velocity will be maintained constant after the accelerating force is removed. As decelerating forces are subsequently encountered the bullet will start to slow down. Maybe the decay to constant velocity takes three feet for a rifle bullet? Or maybe the muzzle blast is still accelerating the bullet past the muzzle? (It is, but compared to decay of acceleration I don't know how much).

A suppressor gives something that is sometimes informally called "suppressor boost", or an increase in bullet velocity compared to the same system without the suppressor attached. In a way, the suppressor baffles act as a barrel extension and continue to accelerate the bullet after it leaves the muzzle. We have to account for this when developing subsonic loads or else they end up not so subsonic after all.

 

[KeithB]

Agree 100% with Ian. If a bullet accelerates after it clears the muzzle it could only be because of the residual gas pressure pushing it from behind. Once any unbalanced force is removed all acceleration stops.

I will offer the opinion that the reason overstressed bullets don't immediately explode at the muzzle but some distance downrange is because it takes a little distance for any dynamic instability to exert itself in a way that increases the stresses enough to cause failure.

The RPM decreases at a lot slower rate than the velocity - I've always thought that one of the reasons some bullet/cartridge combinations expand better than others is less due to forward velocity and more due to rotational inertial forces.

 

[fiver]

yes spinning does influence the bullets terminal performance.
so does the damages to the outer part of the bullet. [rifling imprinting or jacket cutting]
but the forward momentum is what starts the whole process of opening/deforming the nose.
that's why we need a smaller meplat on faster cast bullets when hunting.

 

[RicinYakima]

Which of Newton's laws was that? Acceleration will decay to zero and inertial velocity will be maintained constant after the accelerating force is removed. As decelerating forces are subsequently encountered the bullet will start to slow down. Maybe the decay to constant velocity takes three feet for a rifle bullet? Or maybe the muzzle blast is still accelerating the bullet past the muzzle? (It is, but compared to decay of acceleration I don't know how much).

A suppressor gives something that is sometimes informally called "suppressor boost", or an increase in bullet velocity compared to the same system without the suppressor attached. In a way, the suppressor baffles act as a barrel extension and continue to accelerate the bullet after it leaves the muzzle. We have to account for this when developing subsonic loads or else they end up not so subsonic after all.

Ian, the first law. While most statements only speak of "at rest" or "constant motion", it also includes acceleration and deceleration. The bullet exits the barrel accelerating. In a vacuum, like space, it will continue to accelerate. The air resistance acts upon the bullet bleeding off energy until acceleration becomes deceleration. There is just a nano-second constant velocity with a bullet, as it is always under the pressure from the rear, or pressure from the front.

I don't know squat about suppressors, but I would think that the gas energy inside the suppressor would continue to accelerate the bullet until the air resistance was greater than the "suppressor boost".